#4 - Seth Rollins tears his ACL, his MCL, and medial meniscus (2015)

For a while there, Seth Rollins was the best heel in WWE and, arguably, the best heel in pro-wrestling. As WWE World Heavyweight Champion, Rollins played the role of the cowardly heel to perfection, leaning on The Authority to get him out of his jams and retaining his title by shenanigans in pretty much each of his title defences. It was evident that WWE wanted to make Rollins' eventual title loss a "moment," possibly even a "WrestleMania moment".
And then, much like many others on this list, he got seriously hurt at a house show.
Did you know a top WWE Superstar just wrestled outside the company? More details HERE
In November of 2015, at a WWE Live event in Dublin, Rollins attempted a Sunset Flip Powerbomb on Kane. He came down from the ropes awkwardly, jerking his knee around something fierce. It was later revealed that Rollins had torn just about all of the ligaments and cartilage in his knee, specifically his ACL, his MCL, and his medial meniscus. Projected to be out for six to nine months, Rollins was forced to vacate the championship after an admirable 220 days at the top.
He returned six months later and regained the title at 2016's Money in the Bank PPV, but lost it just moments later after Dean Ambrose cashed in the MITB briefcase that he had just won earlier in the night. Not only has he not held the title since then, but he has been somewhat mishandled since his return from injury. I hope to see him back in the main event scene soon, as he's objectively one of the most talented wrestlers on the WWE roster.