By the standards of modernity, typos were extremely common in the Middle Ages. However, spelling was not standardized, and so they were trying to represent the phonetic of their day as best they could, resulting in some weird results. As an example, a mid-14th century Icelandic manuscript may represent modern Icelandic þér as þier, while a mid-13th century text would represent the word as þer (or þeer, or þEr). This represents a sound change by which long e [e:] became the diphthong [ie]. So, I would not count this as an "error" per se, though it is a place where a change happened in the copying process.

If we discount all of those, scribal interferences go from "extremely common" to merely very common. There are a few types.

1. Misreadings. If the manuscript the scribe was using as an exemplar was partially illegible, used an abbreviation the scribe was unfamiliar with, etc. an error could be introduced. An example of this is in the one manuscript of Beowulf, where line 586 reads "fagum sweordum/ no ic þæs gylpe". This line is not allowable with Old English alliterative verse meter; the two half-lines have to have at least one word that starts with the same letter (or vowels, they all alliterate). We only have one manuscript of Beowulf, so scholars have to guess what the line should read as. Adam Bammesburger proposed that it should be "no ic soþæs gylpe" ("The Emendation of Beowulf, L. 586."). But, for Old English poems with two versions to compare, Roy Liuzza found that 21.6% of lines had scribal interference (i.e. they did not read the same). These would include all the categories I mention here. It's likely that some of these changes were intentional, but many of them werent.
2. Omissions. If there is a spot where two lines in a text share some elements (or even if the same word appears twice in a short space), the scribe's eye can skip the space in between the two repeated phrases, and the entire inside ends up missing from the manuscript.
3. Additions, where a piece is added from an exemplar, either intentionally to reflect information in another manuscript, or unintentionally.
4. Transpositions, where the word order gets flipped to sound more fluent to the scribe.

There are a few ways scribes corrected errors, because they often did notice! There are interlinear or marginal corrections, where a scribe could write a word above where it was supposed to be, in the space between the lines; alternatively, make a mark for where it should be inserted and write the insertion on the edge of the page (you'll need to flip to 6v in the viewer, at the bottom of the page). And in order to delete things that were wrong, it could be scraped off with a knife and replaced, or in some cases a dotted line under the word indicated that it should be ignored, much like strikethrough does today.