Since a few days, many users complain about a bug causing blocking of the system for their iMac under Lion after playback of a video. A difficult problem to reproduce because it would not bug related to a specific video format. For example, several users evoke this incident as well after viewing a video on YouTube only after a video watched from the hard disk via QuickTime or iTunes.
The mouse can always be controlled once after the bug, but no action may be initiated. There are more than the hard reset (long support on the Power button) to restart the machine. An Apple update should intervene soon to Mac OS X Lion 10.7, published last week.
Mac OS X Lion: one million downloads