I believe the reason for the Flair/Hart deal was Flair's departure for WCW (the year where he held the belts for both companies in the same year). And remember, at this time there were only a few PPV's (Wrestlemania, SSeries, SSlam, and Royal Rumble I think) and limited television time compared to today. so having the belt change hands at a non-televised event, especially if it was a massive attendance event.
I am not sure of the reasons behind the Diesel/Backland exchange, maybe just on the spur decision due to injury or some such event.
I don't think any World Championships have changed hands that way since the attitude era, but the IC championship has had several such occurences over the years. Edge was involved with one back in 99 or so. The most controversial title event, however, was the tag championship that the Rockers actually won but due to a re-signing it was annulled and never televised, so the Hart Foundation kept the belts.