I once heard the clink in the machine and (abruptly and probably rudely, in retrospect) shouted "WAIT! I WANT THOSE" and stuck my hand in the coin machine as it was spinning. I counted out probably $3 face value of quarters and gave the guy what I had in my wallet, $20 or so. I made it very plain, I showed him what I took and made an offer of about 6x face which he agreed to.
He then stood in line with the rest of his receipt, looking at the $20, looking back at me with a confused face (why is he paying me more? Should I have not dumped the other ones?). I left, went to another bank to dump, picked up some quarters, went to a store and bought a bag of candy and convinced the bank to open the machine up and under monitoring of staff replace some of his silver quarters with my clad out of the bag. I got probably 20 or so quarters that day.
First and only time I was quick enough!