Anyone who has a substantial and established watchlist on a wiki like Wikipedia probably has some unwelcome additions to it since they added the first items. Every time a page move vandal moves a watched page to a title which is subsequently reverted and deleted, a redlink to the title is left behind in the watchlist of everybody who was watching the page. The result is when I actually checked my list recently to prune some entries, it was full of redlinks to pages like [[HAGGER???????]] and [[George W. Bush on Wheels!!!]].
Needless to say this looks pretty bad to anybody who isn't familiar with the vandalism which goes on an is a royal pain to clear out.
Obviously normally it makes sense to watch both the source and destination of a page move. It also makes sense to keep watching a page which has been deleted if you specifically asked to watch. I don't know if this kind of information is readily available in the MW database, but I'd think the logical thing to do is if a page watched as the result of a page move is deleted, then it gets unwatched. Or possibly this is something that can be integrated into the function of rollback/revert buttons.