My GF and I flew a 3 leg trip 2 weekends ago. Only when we were on our second flight did we realise that we had used the BP for her 3rd leg on the 2nd. If it mattered, NW's computer didn't catch it as the BPs were scanned (seats were the same for both 2nd and 3rd legs so that might have had something to do with it). After we got off at the 2nd stop, I asked a gate agent to reconfirm she had flown the 2nd leg, and called a telephone agent when we arrived at the final destination.

Looking at the BPs, the bar coding seems to be identical expect for the right 3rd.