My favorite is when it pops up right in the middle of entering my password. Happens surprisingly often.
I, as good computer maintenance, regularly clear cookies, but even when I haven't for a while, it usually pops up every session. That could be multiple times in one day.
I also like to stay logged out, and only log in if I want to see a photo or respond to a thread. Just personal preference.
The pop up is annoying but I'm used to it.