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Revenge is a dish best served cold ... and if you're the dude who got kicked in the head by NOFX lead singer Fat Mike -- it's best served by kicking the crap out of the punk rock legend.

A fan named Alex got the chance to settle the score, after getting blasted in the face for putting his arm around Fat Mike during a show in Sydney. Shockingly, Alex took the blame.

So Mike invited the dude to the next show and invited him backstage, where Mike gave Alex a free kick. But truth be told ... Mike kinda wussed out when he saw Alex's boots.

The video is hilarious.