I've had trouble when the last name on the card doesn't match. But there are various interacting rules: for example the one that if the companion fare holder is not traveling, the companion fare holder must pay for the ticket. That has been true for a long time, and at one point (before the requirement that an AS card be used to pay for companion fare tickets) I found that the surname had to match. (One example: my spouse, who did not change her name for marriage, could buy a ticket for her parents who share her last name using her companion fare, but I could not buy a ticket for her parents using my companion fare with different last names.) So they have some kind of checking, plus now they also verify that it's an AS card. But it's not clear to me exactly how much they enforce the must-use-your-AS-card rule or what mechanism they use to verify.