Surprised I didn't see it posted yet....

Deadhead pay.

I know of a GoJet crew that was on 2 of my deadheads, and they had 1 more after that for a total of 3 that day just to get back to domicile. If I recall correctly, they came out of the Texas area (IAH maybe?) to DTW or MSP (can't remember), then deadheaded on my flight to PIT, then next up was PIT to LGA. All at 50% pay.