10. CM Punk
Although we can all look back on CM Punk's brief UFC career as the perfect example of why someone with very little training should not jump into the Octagon, there were a fair few people who were extremely excited and interested in watching him make his mixed martial arts debut.
The connection between wrestling and MMA has always been a strong one, with wrestlers and fighters going back-and-forth between the two and being successful. However, while Punk's run in WWE was historic and is still spoken about by wrestling fans today, his time in the UFC was very much a resounding failure.
His first fight was against Dana White's Lookin' For A Fight winner, Mickey Gall, at UFC 203. Although Gall was relatively inexperienced himself, he completely blew Punk out of the water, taking him down and battering him with ground and pound before sinking in a rear naked choke. Then when he faced 0-1 fighter, Mike Jackson, at UFC 225, he was smashed for three rounds, essentially ending his UFC and MMA career in the process.