Over the past few days, quite a few iMac owners have told us they're experiencing systemwide freezes after watching videos. Likewise, Apple's support forums are full of iMac owners experiencing the same issue.
Although the bug is difficult to reproduce reliably, it's not confined to any one type of video. Flash-based YouTube videos appear as likely to cause the system to lock up as viewing movies from the local hard disk in either QuickTime or iTunes.
During the system freeze users can still move the mouse pointer, but the system doesn't recognize any other form of user input. No error message comes up, and users who have checked their system logs aren't finding any evidence of kernel panics or other system-level errors. Thus far, the only solution to the freeze has been a hard shutdown by holding in the power button.
iMac owners afflicted by this particularly nasty bug have speculated that the 10.7.1 update will be rushed out to address this and several other of Lion's version .0 growing pains. In the meantime, if you're an iMac owner and haven't upgraded to Lion yet, it may be prudent to hold off upgrading for now until we learn whether this problem has been addressed in the presumably forthcoming bugfix update.