I've written several hundred journal and newspaper articles. For decades I've written a piece, had it corrected by an editor and then seen the article published. I write hastily, make mistakes, and am very glad the editors are not like me. So when I began writing for Wikipedia 15 years ago, being bold and just publish came as a shock to me. It was the open peer review that helped me cross the line: it’s all in the open, and Wikipedia is really "the encyclopedia anyone can edit". Which also means that your own mistakes are blatantly clear for anyone to see. Do I make mistakes? Sure, everyone does. And if you have made a few hundred thousand edits on Wiki, it is likely that you have made at least a few dozen mistakes. I can even see a pattern in my mistakes.
Stealing copyrighted text. Well, by stealing I mean: taking two paragraphs from a ten year old text that is straight to the point. It's a deadly sin in academia. It happened to me on Wiki at least twice. It happens to others. I hope we all get busted and corrected.
Uploading images with no clear license. It happens a thousand times a month at Wikimedia Commons. It really happens to me too, especially with photos from 1938.
Writing articles about semi-notable people. It happens frequently, especially if you like minor poets and sculptors.
Adding incorrect photographs of people. Mistaken identities occur but why with female chess players? I really thought this Katarina (pictured) was a young Mária Ivánka.