Catcher plays for both teams in same game
bbc.com
bbc.com
He not only played (and batted) for both teams, but did so in the SAME inning. The game was halted while he was at the plate at the top of the inning with Toronto, and then he came in with Boston to bat with 2 outs at the bottom of the inning. He also ended the game when he came up to bat with 2 outs in the ninth, a strikeout.
[0] https://www.espn.com/mlb/story/_/id/41003854/danny-jansen-ap...
Could have been worse: if he played for both teams at the same time he would have had to hit the ball and then tried to catch it.
I wonder if a footballer has every played both legs of a Champions League knockout game for different teams? (I don't think this could happen because the transfer window wouldn't be open during that period.)