As a Wikipedian, by far the most common edits I have made are fixes to incorrect punctuation. Here are the mistakes I see most often. This list is not comprehensive because it is restricted to the errors I encounter most frequently. I have also tried to restrict the list to things that are universally regarded as incorrect, as opposed to stylistic issues like the Oxford comma.
Some examples were taken from Wikipedia articles, whether they're actual errors I found or they were used in an article as a sentence illustrating an error. Others were made up on the spot.