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1.
To be in the lead in something or to have a streak of success, only to have your success snatched away from you at the last moment, possibly through unfair or unjust means on the part of the person beating you.

Popularised from the game's frequent result in which a 1st place can be snatched away just before the finish line with a powerup.

Can be used as a more friendly version of cockblocked.
"I was so close to making out with this girl, but then Bill Mario Karted the hell out of me by buying her a drink before I could"
by ItsBozzie December 30, 2009
 
2.
An act performed during sexual intercourse. When your partner is about to reach climax you throw a banana peel in (his, her) mouth, and then smack (him, her) in the head with a turtle shell.
I met this chick at the bar last night, and she was pissed when I Mario Karted her while we were hooking up.
by ShadyKev August 14, 2011