One that's gotten me a few times is if you're typing a reply to someone, either in the comments or a PM, then mid-typing you upvote (or downvote) them (or anyone else), it deletes your comment-in-progress. I've lost a few comments that way, one of which took about 20 minutes to write which I just gave up on and didn't post afterward.
I also really don't like that the default language option is "Undetermined". It makes not labeling the correct language the default behavior, which makes a lot of foreign language material show up in my feed. Your own post and 2 of the 4 comments at the time of me posting are unlabeled.