Back in the days of paper tickets, I remember reading an article where a guy got in loads of trouble for the following:

1) He would buy a cheap ticket
2) he would buy a F class ticket for the same flight
3) because he got boarding cards in advance, he would staple the F class card to the Y class ticket
4) he would always arrive to the gate just as the doors were closing, so the GA would not have time to verify things
5) he would then refund the unused F ticket, though he rode in F for the flight