Similar thing happened to me at MGM Detroit around Jan-Feb last year.
I was upstairs playing a dollar five times pay slot. For every dollar coin I put in, it would register four credits. At first I thought it was too good to be true, giving me a glitchy, yet huge advantage.
Naturally some major moral conflicts were spinning through my head. I thought about winning a jackpot only to have the security staff discover the glitch and then start asking me if I noticed anything amiss. The eye in the sky had probably recorded at least a portion of my play.
While debating whether to stay or not, I played through a couple hundred dollars, doing max bet on each spin, but ultimately decided that I didn't want to deal with the enevitable hassle that would come up if I won a jackpot. I'm sure they do some kind of check to make sure the machine's working right. And then, there's the surveillance footage. Wasn't worth the risk - and headache.
So I left the machine and also left it up to somebody else to alert the staff.
...you gotta roll me the tumblin' dice...