Parity. Again.

Annoyance and frustrate. Working on the 4x4x4 cube again (solved it a few times now; last was only the other day). Managed to get all the edge bits into their matched pairs ... But with one pair flipped. It's referred to as Parity. It's impossible on a normal 3x3x3 Cube, but happens 50% of the time with a 4x4x4.

And so far I've not managed to figure out a solution.