I was with a friend at Union Station's Metro stop earlier this week and she tapped her card and went to put money on it. It showed a beginning balance of $6.35 then she put in a $5 and it dropped to $1.35. She told a WMATA employee and he said there's some new scam where people will tap their card once, then leave without tapping it the second time and this causes the money a subsequent user puts on a card to go onto the scammer's card. I don't know if it's actually true but the WMATA employee seemed to think it was.