This slip up takes place actually between 2 episodes. It first is heard in the episode where Henry Blake is going home. Radar gives him a keychain that says "To HB from ROR Korea 1952". Now...in an episode a year later when Col Potter is in command, he is seen playing Father Time at a New Year's Eve party the camp is having and on the banner it says Happy New Year 1951 and Potter says "Here's farewell to 1951".

Oops...