do you have a common name? I used to have a very common last name. On an AA flight from DFW to San Jose, CR apparently some other lady with the same last name had checked in as me, changed my seat away from the group I was with, and tried to change the FF number on the ticket. When I tried to board (with a BP in my name with my old seat) it came up that I had already boarded and the seat on the BP was different than my current seat. I was pulled aside until they could pull her off the plane, verify who she really was, then get me a seat.

...She noticed the FF number was different but not the first name! That lady had been stranded in DFW for a couple days.

You'd think on an international flight someone would have paid attention during checkin.