I was lucky because I barely got any spoilers (Because I had and have no friends lol) but two occasions I remember are
When I was in the middle of reading the Goblet of Fire I sat down to watch my tv and I was flipping through the channels (wow, I haven't had cable in years, that makes me feel nostalgic for some reason) and they were playing a clip from the Half Blood Prince where Ginny kisses Harry in the room of requirement so I knew they ended up together.
When I was hanging out with my nanny right before I went to watch Deathly Hallows pt. 2 in theaters she was watching a clip of HISHE Deathly Hallows pt 2 and I basically got a preview of the movie