I'm surprised this doesn't happen more frequently, actually. Especially with things like poker tournaments that pay cash, where you can pretend to be a spectator and know exactly how much people are walking away with.
It's one of the reasons I pretty much only gamble on the Las Vegas Strip, where I assume there's a small army of plainclothes police officers around me. (If there aren't, don't tell me ... I'd rather be lied to in this case.
)