My best one is when I was working at a retail store, and was using my phone for music over the store sound system since our satellite radio was down.
I forgot that I played on my break and went to change a station or something. I opened my phone and had the startup sound blast through the store. One guy I worked with plays, and I heard him bursting into laughter while I was in the office. Had some confused looks on customers faces as well. Good times.