Craziest In Game Experience.
I was playing a Dwarf who was considered an outcast due to my heritage. To prove my worth the Taskmaster had set me up as the leader of our party to go and scout for the army. After several sessions and many group experiences where they learned that my character cannot swim and therefore made crossing rivers a very difficult task, my character was knocked unconscious. He was taken prisoner and replaced with a doppelganger. My DM had me play as the doppleganger to prevent out of character knowledge seeping into the campaign. Suddenly my character could swim, was worse with an axe, and several other oddities but my entire party continued as if I was the same person.
It took an entire session before anyone learned the truth about my situation. For myself and the DM it was hilarious, for the party, well they nearly died several times from my “mistakes”.