Maybe not 500, but two of my friends may have hallucinated the same thing at the same time.
It was our freshman year of college, and we had just gotten back from somewhere at like 2 am, so we went to bed. Their room was set up where one of their beds was at the front left corner of the room, his head at the other end of the bed from the front, and the other bed was at the back left corner, his head in the corner. Anyway, they go to bed and talk for a few minutes and suddenly they hear a voice say "BRAINWASH" in this like creepy announcer voice coming from the space in between there two beds. One of my friends is like "Dude, did you just hear that?" and the other replies "Wait, you heard that too. What the heck?". They ended up sleeping in my room that night, and we spent all of the next trying to figure out how that happened (the room was right next to the staircase), but there is no way it came from the staircase. To this day, we have no idea what happened.