Years ago I had a problem because of one boarding pass. I was on the old red eye flight 777 from TPA-LAS-PHX. We left TPA late and when we landed in Vegas they told us or flight to PHX was on a different AC. When we got out of the jet way we found out the PHX flight was leaving any minute from the other B side and we where in A. Everyone ran to the gate. I had left my stub in the seat back pocket and the gate agent was not going to let me on. So for any irregular operation I'd prefer to have two boarding passes in hand.