Blogs that play music!!!!
Especially when I am mindlessly surfing because I am trapped under a feverish child and the darn music starts playing and startles her into waking up. I never re-visit blogs that play music. I often turn my speaker off so I don't know usually, but DH must have been using my laptop recently and turned the speaker back on.
It's happened to me twice this morning :( I know some people love that feature, but I find it one of the most annoying things on the internet -- right up there with internet ads that talk out loud!