Hmmm never had that happen. If I put in the wrong type of coin it has always just fallen through to the coin return. And damn did I try everything. Especially electrical box knockouts - my dad was an electrician and as kids we always asked him for those. He had no idea why we wanted them, and didn't ask. We used to just feed them through over and over until we got a credit. It worked at the local 7-11's narc and bad dudes machines, as well as the popcorn machine at the laundry mat.
That makes more sense though, having it hold the coin. My coin door is missing pieces so that's why it eats everything.