e.g. The video associated with this item’s been removed:
Though not strictly a broken link it has similar implications (i.e. takes the user somewhere irrelevant), it looks like it’s still possible to detect this programmatically: http://stackoverflow.com/questions/1591293/programmatically-determine-whether-a-youtube-video-has-been-taken-down
When detected it could trigger automatic cleanup, notify contributors so they could amend, or be added to a queue of maintenance work (in the same way questions with no options are listed currently).
This should probably live at the bottom of the priority list for suggestions as it’s a very minor inconvenience & quite a rare scenario.