My two favorites are:
When my old hippie band was recording our album in NYC, Luscious Jackson were doing post-production on their latest album (Fever In Fever Out) in the studio next door. We ended up hanging out in the lounge when Jill Cunniff (lead singer) came in to hang out. I asked her to teach me a song from her side project album (Kostars) and she did. She then had us drive her home, but had us drop her at the corner so we wouldn't know where she lived.
The other was in 99, the Chili Peppers were having a fan club only show at the top of the World Trade Center. My brother and I went down there, and he got us in pretending we were working as crew for VH1 (my bro worked for CNN as a sat truck driver at the time). When the real VH1 did arrive, they had us help them setup the cameras and lights and then got to watch them interview Anthony and Flea, and then I got to shake their hands and shoot the shit with them before the show. One of the best nights of my life.