It happened to me in 2011. I had a rtw ticket starting in Amman, issued on-line by AA's oneworld booking tool at the ex-Amman price while I was physically in Rio de Janeiro, but using a US credit card. There was no contact with a travel agent or ticket office anywhere. When I wanted to change the routing I initially had no trouble with AA's round the world desk in the US, but a few days later they called me and said I would have to pay several thousand dollars more, but I could change the dates without any trouble. They explicitly mentioned that the on-line ticket was considered US issued because of the credit card. They said that the on-line tool had made an error by giving me the Jordanian price, but since it was, in their view, an AA error they would honor the ticket with the original routing. Perhaps there would have been no problem with a foreign issued ticket with a US credit card if it involved a travel agent or an overseas ticket office. And perhaps it was just an unusual case of bad luck. By the way, I vaguely remember posting about this problem on FT at the time, but I can't locate a reference.