On Programmers we have the following question:
Which had been closed as a duplicate of this (now deleted) question:
The first question was closed on May 6 '13 at 15:24, while the duplicate target was deleted on May 27 '13 at 14:57 (by a single moderator vote as it happens, but that's not the point).
By allowing the deletion we are creating broken links for the vast majority of users who can't see deleted posts.
At the very least there should be a warning presented when people try to vote to delete and a message to check the links on the right. It could be that these other questions need to be deleted as well but they should be deleted first before the target is deleted.