in my forum we delete a lot of threads and also we delete a lot of users (if they wish so).
So all the time, if a deleted thread or user (or even banned users) Google trys to index that page but isn't allowed to (Google Webmastertools are complaining about this stuff a lot!).
Also, if the privacy setting of a user profile is set so only registered members can see the profile, the same stuff happens.
Maybe there is a way to detect those links to deleted threads and members. Would be great since this is a problem everyone is dealing with (i guess).