Yep, yours truly walked in on some random guy trying pants and a shirt on. I left the room minutes before after trying on a pair of pants, checked out and realized I didn't have my keys. I went back to the dressing room, the indicator thing on the door was green (aka, OPEN) but I knocked, just to be sure, said "hello" and slowly started opening the door when I see a bare arm quickly reach for the handle and a voice say "Someone's in here - I guess I should lock the door". Ya think?!?!
Turns out, one of the workers had already been in the room, found my keys and they were waiting at the register for me. Lesson learned - knock on the door harder and speak louder before you open a dressing room door but seriously people, lock the door! :-)