What happens if you get caught attempting to use counterfeit currency unknowingly BUT you can't remember how it came into your possession?

If, for example I'm making a purchase at a retail store and they realize that a $20 that I handed them is fake... I guess they would either call the police or just question me themselves... but either way, if I truly didn't know it was counterfeit, what happens? (? 1)

If I can provide a source from which I got the bill, or if I say it was just given to me by a friend, what happens? (? 2)

And if I tried to deposit the bill into my checking account and the bank catches it, what happens? (? 3)

... I am asking out of curiosity because this almost happened to me, except I realized the bill was fake and threw it away before spending it (I was handling my money with wet hands and my $20 bill's ink started bleeding! It was a crappy photocopy!). I couldn't for the life of me figure out where I had gotten it from. The only ideas were from a tiny convenience store/corner store in a low income neighborhood that may or may not have illegal gambling machines in the back, or from the woman who's daughter I babysat that lives in the building above the convenience store (I think she might be involved some sort of illegal activities like gambling or drugs)... those are the only sources that are shady enough for me to consider the bill cam from but I just can't say for sure. If I accidentally used it and got caught I wouldn't want to say it was the woman, because if it was from her I would say without a doubt that she didn't know it was fake either.