Apollo 8, 10, and 11 had all already used "free return" trajectories, which would have caused the spacecraft to "slingshot around the moon" and return to earth

Apollo 8, 10, and 11 had all already used "free return" trajectories, which would have caused the spacecraft to "slingshot around the moon" and return to earth in the case of main engine failure. Mission Control would not have needed a cold call from a hippie physics student to remind them they could try that with Apollo 13. This could be a garbled version of some other piece of critical advice coming in a cold call. But it's also possible that some hippie grad student thought he came up with the idea of free return trajectories on the fly, and assumed that long hair was the reason he never got credit for telling Mission Control what it had already known for years.