#1 Failing to cash-in Money in the Bank contract
John Cena is on the list of those wrestlers who failed to cash in their Money in the Bank contracts successfully. It was a major embarrassment for him, as he had a failed cash-in attempt against CM Punk.
Cena won the contract in 2012. He decided to do something unconventional and intimated CM Punk beforehand about his cashing. This was done to maintain Cena's heroic image in-front of the WWE. However, it cost him the match.
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Big Show interfered and caused a disqualification. Cena lost the opportunity to become the World Champion. This indicates that Cena's decision to inform Punk beforehand about the match was not a sensible decision. It was a learning experience for John Cena.
The next time he won the Contract he would use it to win a title, however, the failure to cash-in the title is something that will never change. This is the one blemish that Cena will have to live with forever.
This shows that even a legend of John Cena's stature has several flaws in his career. However, if Cena wants, he can alter those mistakes and prove to the world that he is Champion material and break the record set by Ric Flair of 16-time WWE Champion.