I was sneaking and stealing stuff in the temple of Mara and accidentally stole a tankard that I didn't want. After I dropped it I heard someone upstairs say something like "you should be more careful with your things". He then walks down to the room I'm in and puts the stolen tankard back into my inventory. I was undetected the entire time. The neat thing about it is that when he put the tankard back into my inventory the 'stolen' tag was removed, It had become my property!

After a Frost Dragon decimated every npc that was outside in Riverwood (except the kids), A thief spawned inside the Riverwood Trader. He's there every time I go, and he just runs around looking suspicious while everybody just ignores him.. The last time I went there he was actually cooking! When he noticed me he turned around and was like "Damn! he caught me being unthief~like. Do something evil, Quick!" He draws his dagger and puts it away again, then resumes hes running around routine.

Mine is even worse...after doing a certain quest involving some mad guy living alone in the northern wastes, where he, well, died, I returned there much later, while I was exploring the surroundings, then I took some object flagged as "Stolen" (which later I even changed my mind and dropped it back). Well, a while later while I was in Falkreath, some thugs attacked me, and when I killed them I found a note about that guy sending them to "teach me a lesson"....

Dead people sending thugs to kill me. Creeepy...