I have been annoyed when I had something to add to at thread but determined for other reasons that it was not a good time to bring it up. For example, if the thread has moved on past the post I wanted to respond to, or I knew that what I said would cause an instant hijack.
I’ve also occasionally felt bad for not posting in a “in memory of” thread when I thought well of the poster but can’t think of anything to help the grieving. Though, at least in that case, I do eventually think of something.
Finally, I will admit a slight twinge when I come up with the perfect joke and someone has already made it in a much, much better way than I can. Mostly this is because thinking of a clever enough joke for this place is hard enough already. I can’t just wing it with something out there and silly.