#2. CM Punk
CM Punk might be the single biggest and high-profile walk-out in the WWE's history (some might argue that it's Stone Cold Steve Austin) when he simply didn't turn up to Raw or SmackDown as scheduled in 2014 and then was never seen since.
Punk later infamously revealed the full details on Colt Cabana's podcast stating that he was suspended for two months after walking out on the company in January and wasn't contacted by WWE after the suspension was over, eventually being handed his termination papers on his wedding day. Something Vince McMahon actually apologised for.
Punk's departure was also allegedly spurred on by health reasons as he alleged he was working through an untreated MRSA infection. Something that would go on to be the subject of a now infamous law-suit.
It's been five years since CM Punk walked out on the WWE and he literally hasn't wrestled since then. He did begin an MMA career that unfortunately saw him lose two fights, but seems to be transitioning towards a commentator role.