I've never met anyone that was an idol of mine that I had a bad experience with.
However, my best friend growing up had his go so bad he ended what was nearly 20 years of fandom right then and there. He worked for one of the big shoe stores and had won a contest to go to the All Star festivities for the NBA. He met a lot of current and former player and coaches and had a blast. Until he met Michael Jordan. Being a sneaker head, my friend owned originals or rereleases of every shoe Jordan ever had up until that point. It was something like 30 shoes if you counted the different colorways. Posters, jerseys, trading cards, memorabilia of all types adorned a room at his apartment. He sees Jordan and decides to just kind of hang out until he is done talking and just get a quick meet and greet, no photo, no autograph, just to say hey. Jordan ends his conversation and my friend steps up near him and Jordan basically asks him who the f**k he is, what he wants and acts like an a*****e. My friend just tells him that he was a big fan and it was great to finally meet him. Jordan basically says, "yeah, whatever" and walks away.
He got home from the trip and started selling off his collection of shoes and all things Jordan. He had heard the stories and assumed they were just overblown or didn't happen. Nope, Jordan is a c**k in real life.