Gawaine made a dodgy bet with the Green Knight. Challenging the court at Camalot, the knight offered to let someone cut his head off if he could then have a go in return. Thinking he was onto a sure winner Gawaine took up the challenge and cut the knights head off. Unfortunately, he did not know the knight had the power to walk around without his head attached. Then it was the knights turn for a bit of head chopping. Actually, there is a year between the two events and some questing to be done, as happened in those days. It also gave the knight time to fix his head back on.
