When flagging a comment on an answer scored -3 or lower (so it's grayed out), if you move your mouse off of the answer/flag dialog the flag dialog grays out as well and hides under things.
In this example on Puzzling.SE, the bottom of the flag dialog snuck under the spoiler on the answer below. It's a little more drastic when the answer is at the bottom, though, such as in this example on Scifi.SE:
In this case, the flag dialog hides under the place where you write your own answer.
I assume this is not intended behavior.