One of mine, forum-related, is when someone comes and offers some advice on a topic as though only they have had this insight or depth of experience, when one of us has already said the very same thing three or four posts back.
I suspect I am probably super-sensitive on such things, being a woman of a certain age, and having spent a lot of my earlier career having to prove my worthiness to be listened to. (Think "Life on Mars" and the regular - male - coppers' attitudes to the "plonk" - female copper. I started work as a software engineer in 1980, and had to correct people's assumptions that I must be my male colleague's secretary for most of that decade, and beyond at times, although things were definitely improving by the late 80s.)