I understand that WW1 artillery got more and more sophisticated through the war, with well coordinated creeping barrages working closely with infantry.
How does every gun ensure that their shells are on target?
For a single gun (or maybe even a few guns), spotters can be used to feedback accuracy information back to the gun to make targeting adjustments. But in barrages, can you even tell which shell came from your gun?
Assuming they cannot use feedback during the barrage, these guns will be all aiming at their targets based on ballistics calculations alone. Do the guns need to be calibrated to ensure that calculations are accurate? I imagine that different factors like wear and tear of the gun or variations in propellant would need to be adjusted for.
If for some reason there was one gun that was consistently firing short (and not corrected for), then throughout the creeping barrage this gun will be a threat to its own infantry. Is there a procedure to ensure that this does not happen, or if it happens then to stop it from continuing?
Most artillery fire is about creating an area of fire vs striking point targets.
Which is to say getting a round within bursting radius of the target is considered a hit. Deviation from point of aim was expected and even desired to some degree.
Within the confines of this dynamic, artillery was fired as a battery (guns sharing a fairly small geographic area) relying on math to get on target. Generally when firing for an observer, one gun is fired from the battery, so the observer may clearly see it's fall or deviation from target. He then passes back corrections to the battery. Then another round is fired and the process repeats.
Once the spotting round impacts close enough, then the entire battery will "fire for effect" and shoot the desired fire mission. The non-spotting round guns would adjust their aim off of the known point of aim for the firing gun (if I'm at the same range, but firing 50 meters to the right of the spotting gun, I would adjust the traverse of my gun accordingly).
Sometimes this adjust fire process would be completed in advance for a major operation, without the fire for effect. This was known as "registering" guns, and in effect pre-aiming for a later fire mission (The guns might not even stay on that line of aim, simply the exact elevation/traverse would be recorded for future use). This would allow for massed and accurate fire without the warning of spotting rounds on the day of the attack, but did serve to let the enemy know something was up.
An artillery barrage is not about hitting a specific target but about carpet bombing an entire sector. Think of this as defining squares of rectangles of X hundred meters by X hundred meters, sometimes entire sections of the front. You just flood it with shells in an undiscriminating fashion. You still use observers for corrections but basically move the barrage back or forth as a whole. Individual shells do not count.
Since this is WW1 you still have less technically evolved explosives, meaning you still get a lot of smoke making accurate assessment of the shelling difficult. In addition in many cases they were used to prepare an offensive and bombardment may rage for several hours, even days.
Good answers thus far, I'll add a little bit extra information to complement what has already been put forward.
So due to the industrial production of both guns and shells, there is a uniformity across the board. One can assume from this that exact calibre rounds from exact calibre guns will land in pretty much the same place with relatively small error. With hundreds of guns firing down the line it would be difficult to determine which round was yours, however if you conducted the right calculations you could time the round in the air and make a guess as to which impact was your own.
The common use of indirect fire meant that the guns would be layed using a lining plane sight with information gained from the Battery Command Post including the elevation angle so that different guns with different barrel wear could effectively and accurately fire.
It wouldn't be difficult to notice a gun that was firing short for a couple of reasons.
There may be a problem with the batch of ammunition, but this would be noticed among many guns and would be removed from service as quickly as possible so as to not impact your own troops.
There may be damage to the gun itself, but if the crew are checking it regularly for damage (which it would be doing on a daily basis), then the gun would be removed from the line to stop short shells and the potential for a large explosion on the gunline if a breech explosion occurred.
Hopefully this helps to answer your question!
Strong, Paul, Artillery in the Great War, (Barnsley: Pen and Sword Books, 2011)
Found this and remembered your question. Have a look, this illustrates how WW1 artillery barrages worked.
Author is an US Army engineer who regularly writes about tactical scenarios. This should complement my previous comment.
Edited for further explanations.
https://angrystaffofficer.com/2016/07/01/anatomy-of-a-world-war-i-artillery-barrage/