if that happens all you have to do is make sure you have a copy of the TXID and send a support ticket and they will fix it for you at p2pb2b , it even happened to me once , all I had to do was send them the TXID number and they fixed it
I did everything and also made a screnshot, and indeed they said there was a problem and until now my deposit still hasn't arrived. but OK, because the supporters are very good at responding, and surely this will arrive