A lot of museums are themselves very long-running institutions which have undergone changes in personnel, management, ownership, and collection cataloguing methods.

To give an example, a museum I went to regularly as part of my MA holds a collection of swords, these swords were found over the course of the last 200 years or so in excavations or by turfcutters and, before computers, were given a number, which was tied to the item with a card on a piece of string. The museum staff also filled out a card with a small drawing and a paragraph of description. One such sword was given to another museum in England (at that time Ireland was ruled directly from London) but they lost the tag. So when I went to the museum in Ireland they had the card for a sword they didnt have. I went to the museum in Britain because I knew they had several swords of the type I was interested in, and lo and behold they show me this thing that looks like what I'm studying but they don't know where its from other than it matches the general type from Ireland, I tell them to contact the Irish museum and the artefact number I think the sword should have, and a mobth or so later its up on their online catalogue with the correct information.

Other times, usually for small artefacts, a museum employee takes some items into their office to study, they're writing THE ULTIMATE work on La Tene yellow glass beads or something. Their work gets delayed, they have a museum to run, other articles to publish, they take a sabbatical or two, the beads are loose in their desk drawer but its cool because they can tell them apart and they know which one is from which site nd all that.

Then they die.

Then no one knows where thing X Y and Z are because they're not in their proper drawer and they've been separated from their containers with the info about what they are and where they're from. Maybe whoever cleaned the office popped them in some other spot along with the papers and sweet wrappers from that curator's desk drawer. Then 50 years later nobody has done any work on La Tene glass beads so nobody cares until some intern looks under a box and finds these things that nobody who works at the museum has ever seen because that old researcher's colleagues are all retired and/or dead too!

Great question! Seems totally illogical, right?

So let's start with some basic information and statistics. Museums have X amount of space to display things - there are limited walls to hang the paintings, limited spaces they can regulate temperature, etc. And, museums have curatorial narratives (the story) and the objects on display need to fit with this, which could reflect temporary exhibits that year or because objects can be "problematic" based on their provenance. For these reasons, and others, museums only put on display about 5% of what they own.

So imagine a big museum, with lots of stuff on display. Now imagine that's only 5% of what they got. Everything else is in storage. Thats a lot of storage!

Museum storage is really interesting and overwhelming. Where I work, there are some hallways with glass cases of pottery and other areas filled with specimen jars. I've been to other museums which have converted old bathrooms by filling them with shelves, or by just taking palettes and stacking huge books on top of them in former stalls. The more valuable stuff is usually kept in better boxes and better documented. Here is a great photo of the Smithsonian's specimen storage. And here is what media storage looks like (this is a more expensive storage system than just boxes in a bathroom, clearly).

On a very practical level, there is so much stuff that things are fairly easy to loose. Sure, the collections managers and registrars should have everything documented. And most of it they do, but what if one thing got misplaced? What if something got put away in the wrong box and no one looked at it in fifty years? What if someone put something back in the right place but they didn't realize what they really had? Those are the discoveries.

The root of this problem, in my (professional) opinion though, is a lack of available staff. Museums are generally short staffed and the pay leaves a lot to be desired. Your favorite museum is probably run by someone overworked. They don't have time to just, pick a subset of their giant collection and comb it looking for something suddenly relevant or something misplaced. Therefore, the "discoveries" in the collections are kind of like a game of chance. If an inbound research request comes in, and the archivist goes looking at artifacts in a box that hasn't been opened in 50 years, only to find, for some reason, a valuable document was hidden in there the whole time, then thats a cool discovery.

These discoveries are cool, but the museums also need to remain financially okay and get visitors in the door. In addition to finding a cool lost item, it behooves them to publicize what they found and make a thing of it.

I wouldn't say this is a common occurrence, by the way. There are 35,000 museums in the US and 2,500 in the UK. Now think about how many artifacts that means they each have (this is obviously a huge variety, some museums might only have 1,000 while the the Smithsonian - the world's biggest museum network - has 155 million). So the occurrence is actually quite rare, though you're right, it seems like you hear about this often enough.

Interested in learning more about working in museums? Check out /r/MuseumPros!

Sometimes, when items are archived, the labeling can be misleading or outright wrong. When I was working on my dissertation, I was doing research at the Library of Congress. My topic involved the period surrounding the Spanish-American War, so I had requested some materials from that era, mostly involving Theodore Roosevelt. Often, when you do research on an individual or a specific time frame, you'll get a large box with the documents you need plus lots of other extraneous papers on similar topics. It's always a good idea to give these documents a once-over, as historians are always looking for ideas for a future journal article or book, and most of these boxes won't have been physically examined in many, many years, if at all. When I looked at the catalog entry for this box, it stated that it included a folder entitled "Articles related to the assassination of William McKinley". While this wasn't my topic, it certainly looked interesting enough for a once-through. I had assumed that these would be governmental and diplomatic reports related to the assassination. When I received the actual box, however, what I found was much different. I pulled out the folder initially confused, because all I found was a black wallet and a couple of business cards. It took a minute, but I realized that what the label actually meant was that these were articles that McKinley had on his body at the time of his death! They're certainly the most remarkable historical objects that I've ever accidentally run across during research, but it shows how something can be poorly labeled in a way that while technically true, doesn't really reflect the items within, causing them to get forgotten about.

To add on to the answers you've already gotten, it's also the case that museums, historical societies, etc. add material to their collection sometimes as a matter of course, and not everything that's added is painstakingly researched. In my state, the state historical society has a state newspaper collection (both bound volumes and on microfilm) that they add to because the state press association collects and gives them newspapers. No one at the historical society has actually read all of those, so the tag in the catalog lists issues of the Springfield Daily Whatever from 1898-1997 (I'm making this up, but you get the idea) that someone doing research will have to go find the film of, put in a reader and manually look through. As far as I know this issue extends to ancient manuscripts and even tablets/cuneiform stuff as well -- there's lots sitting around in museums and archives that just hasn't been looked through.

This can have various reasons. Museums usually aren't just there to display artifacts. They are often institutions for historical research or at least linked to research institutions. They don't just display items, they also collect items, data and do research. While they may have a lot of artifacts in their possession, that doesn't mean that these are all analyzed and that the proper context has been determined. So it's not exactly that they aren't catalogued, just not intensively studied yet. This takes a lot of time and resources.

The most obvious way a new item is discovered within the collection of a museum is that the museum finally found the time, resources and right historian to give context to a specific item. This happens all the time. There are way more artifacts than competent historians or funds. History as a discipline is notoriously underfunded, as is true for many of the social sciences. That's why you'll have University professors have their students do a lot of the grunt work, haha. There's also a less obvious way as to how these discoveries happen. At times, the academic concensus regarding a certain aspect of history changes. This changes the context and may put already known items into a new light. This will also be put forward as a discovery because of the newfound significance of these items.

You have to realize, there are A LOT of historical sources that simply have not yet been uncovered. This is especially true for the vast amount of written sources. We have them. We vaguely know what they are and where they are from. We simply haven't read or analyzed them yet. There are a lot of historical institutions who are currently working on making written sources available online to make it easier for historians to quickly read and analyze them. This has actually been an almost revolutionary change fairly recently for history as an academic discipline, the disclosing of previously hidden away sources through technology that makes it easier to translate them to digital files and then share them. It's still ongoing.

Let me give you an example. When I was still studying at University and I wanted to analyze medieval Almanacs, I had to first get permission from my department head. He then contacted the department head of the historical institution that had these sources in storage. I then had to take a two hour train ride there. They had me pass a few security checks before I was allowed to enter. I then had to wear a mouth mask, gloves and put the sources on a pillow before opening them. I also wasn't allowed to study them for more than four hours at a time. While this still persists to this day, it's far less common. The students I taught simply had to go to an online database and download their sources in order to study them.

So, especially with written sources we've entered an era of rapid disclosure and ease of access which leads to more discoveries. It's still problematic for many objects to find the right historian and the right context though. However, even there we have digital imagery and storage. So truly exciting times for historians. Still, like I said earlier - and like mentioned by others - it's still very difficult to get enough funding and resources to quickly analyze these sources. We often heavily rely on volunteers or history enthousiasts to do a lot of the grunt work, but they don't always have easy access to these sources or knowledge of the proper methodology to analyze them.