[ct0]

This is my pet peeve. Most of the time I notice this when im interested in a reddit thread that's deleted and I lose the ability to review it.

[lucb1e]

This, plus any sort of live data (think stock market, tweet feed, etc.), plus any sort of form input (input fields should survive since like Firefox 3.5, but then people discovered you could also dynamically build forms with Javascript and the rest is history).