I'm glad you got your $500 back - that would have been very disturbing.
I had two variations on that (for a smaller amount):
1) I was playing video poker at the Palms a number of years back, put a $100 bill in the machine. It took it and nothing happened - no credits, nothing to cash out. I hit the service button, and they checked the money drawer and must have looked at video, because after a few minutes they gave me a new hundred. And yes, I put it in that machine and continued playing.
2) I was at our local casino, Mystic Lake, playing, got up for some reason (don't remember what now) and left $100 in the machine. Needless to say, by the time I realized it and went back it was gone. Oh well. Gives you a sinking feeling, but I decided to just wish whoever got it luck with it and went on playing at a different machine.
I try to be more careful now...
;-)