Jim Marshall
Marshall was a brilliant defensive end for the Browns and the Vikings. However, he dropped one of the biggest clangers in NFL history. Imagine recovering a fumble and returning it for a touchdown. Nothing seems wrong with this sentence on the face of it but Marshall made a key error (via History.com).
Somehow the defensive end contrived to run the wrong way up the field back to his team’s end zone. He charged unopposed for 66 yards in the wrong direction. Nobody could believe what they were watching because it was so ludicrous. In the end, Minnesota still won the game but this was a career-defining memory.