My little brother recently started his own phone line w/T-Mobile & got an iphone 6 on the payment plan (put like $150 down & had 2 years to pay the rest). Well, within a month I had him switch over onto my plan (lowered our cost each by a few bucks) & when we went to the store to switch him over, the guy told us they'd add his balance on the phone over to my account so he could continue paying.

A week later he gets a bill in the mail saying he owes the full balance on the phone. I call, dont even get angry, just ask that they add the balance to the bill in installments as planned. The dude on customer service says "well, I don't have the option to do that...& since his old account is technically closed...i guess you just get it?"

Apparently because of their fuck up (weren't supposed to have the line deactivated before the payment stuff was sorted or something) he had his remaing $600 or so wiped clean.