Within three minutes of the free return burn, FLIGHT Glynn Lunney suddenly notices that the wall clock in the MOCR is off by a large amount. Once RETRO Tom Weichel provides a countdown from his retro clock indicating the time left before the burn, Glynn suddenly becomes a
lot more animated over the loop.
It was shortly thereafter determined that the wall clock had been improperly set by the comm staff, and that both the spacecraft and the retro clock were properly synchronized to the burn time.
Link:
https://apolloinrealtime.org/13/?t=061:27:28&ch=19As an addendum, later before the PC+2 burn, FIDO Bill Stoval (also in the MOCR at the time of the free return burn, sitting next to Bill Boone) reminds Bobby Spencer to check the wall and retro clock to make sure both are aligned - "I'd hate to get zapped on that one again."