I've been on two flights in the past two years (one on NW on on Frontier) where a psgr had boarded a plane to the wrong destination. On NW, at least they caught it before pushback; on Frontier we had already taxied for a while before we ended up going back to the gate to let a woman off so she could go to some city other than ABQ. Given that people get on flights they aren't even supposed to be on, it isn't terribly surprising that you were able to get on using an out-of-sequence BP.