In the original Superboy & The Legion of Super-Heroes stories, Saturn Girl had placed a post-hypnotic suggestion on Superboy to forget everything about his future that he might discover while in the 30th Century. For example, the fact that Supergirl existed was wiped from his mind every time he journeyed back to Smallville.
I mean, think about it -- Batman complained about one little mindwipe, but Superboy had a constant mindwipe happening to him after every adventure with the Legion! Those bastards!