My computer froze up when I responded to this, so my apologies if my post appears twice; couldn't tell if the first "send" actually went out.
After reading all the information and posts on this, I believe I have the solution to the puzzle. Clearly, when you weren't looking, someone put money in your machine. They probably were having trouble finding the right slot on their own machine, or were doing it in the way of congratulating you on your win.
I'm sure everyone on this list has had this happen to them 
Seriously, I'll vote for computer error - bad chip, bad programming, or something. Bad machine design if in fact the attendant can add credits, purposely or inadvertently!
--BG
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