Identical or nearly identical characters. It is one of my “pet peeves” when I’m reading. Even if the dialogue is different–even if they wear different clothes or have widely different physiques–they end up describing a scene in the same way, with the same words, the same attention to certain details. I don’t mind it if there is an overall shared sense of humor (an author will likely have personal preferences and write accordingly) but there are still some things one character would say, while another wouldn’t, and if we can’t tell at a glance…an author has a problem.
But here’s a good post on how to fix this. 🙂