Something kind of similar happened to me but the responsibility was a lot more vague.
Person X was in charge of collecting money for something everyone in the organization needed to contribute to. I rarely come in contact with Person X. I see Person Y quite often, and he said he was going over to find Person X later that day to give her his share. I said "Oh yeah, I need to do that too." Person Y volunteers to take my money to Person X for me, so I than him and hand it over.
Days later, Person X contacts me and says she hasn't received my payment yet. I tell her Person Y was bringing mine to her. She said Person Y had paid his, but not mine. I ask Person Y about it and he looks a little confused at first, like he doesn't remember me giving him my money, then his expression clears and he says, "Oh yeah, I gave her your money."
I trust both of these people completely. I would bet just about anything that neither of them was out to steal my money.
So one of two things happened: either Person Y thought he gave Person X my money but he never did, or he did give her my money but she forgot to mark me as paid as well as him, since I never actually handed her money.
Either way, it was my responsibility to get the money to Person X, so I paid her. It was only $20. I might have been able to sort it out by getting them in the same room together and talking it out but it wasn't worth it.