Supergirl/Wonder Girl 6: Kara can't remember the LSH, but Conner has to remember the Ravers, so...

Now, as is often the case, I'm basing this off memory, and don't have the issues in question right handy; but in the old, pre-Crisis, pre-reboots, Adventure Comics era of the Legion of Super-Heroes, Superboy went to the 30th century on a pretty routine basis. Supergirl later joined the Legion as well, but she was coming to the future, from a period several years after Superboy's visits--Kara didn't become Supergirl until well after Kal-El became Superman.

For example, let's say Superboy was leaving his home time of 1959 to hang out with the Legion in the year 2959. Supergirl could be there in 2959 as well, but she would've come from her home time of 2969 or so. I'm not sure about Supergirl, but Superboy and the Legionnaires were also aging at roughly the same rate: as he became Superman, the Legion became adults as well, and usually that's how they would meet. Rarely, if ever, would the adult Legion run into Superboy, or Superman see the teenage version of the team.

And people wonder why DC needed a Crisis, multiple reboots, etc. More tangents after the break!

Getting to the point: Supergirl had a post-hypnotic suggestion placed on her, not sure by whom exactly, so that when she returned to her time, she wouldn't remember any details of the future that she shouldn't. Like her death, for example. Actually, her death almost makes the suggestion make sense retroactively. But it is a little dicked up that just about everybody else gets to go back and forth in time and not have their memories wiped.

The exception, that's in the same boat as Kara: Hal Jordan, of all people. Early in his career as Green Lantern, the 57th or 58th century (58th, sorry!) abducted Hal a few times, brainwashed him into the identity of Pol Manning, and used him whenever they needed their superhero, putting him back none the wiser on more than one occasion. (I'm not sure how that worked: Hal started to notice he was having memory lapses, blackouts, and investigated and figured out what had been happening to him. But, if they're yanking him through time, shouldn't they just put him back right when they got him? Hal might wonder why he's tired and covered in bruises, but it's Hal, they could've kept that up for years more.)

Eventually, the 58th century Solar Council stopped using Hal, and accidentally got Salaak once, to fight the anthropomorphic squirrel descendants of C'hp...and this is why I won't stand for anyone badmouthing Zero Hour, since that event apparently took out the whole mess. (Hmm. This also means Salaak, of all people, like Peter in Heroes, has a girlfriend that he left in a future that has now been seemingly erased. Like Peter, Salaak isn't about to open that can of worms...)

Again, based off memory, Kara had the post-hypnotic suggestion, but I don't think Clark did. Also, I believe Kara was trusted not to spill the beans about what she knew of Clark's future: "Well, you grow up and become Superman, and Lex Luthor's still a jerk, and that Lana chick is still following you around..."

Previous episodes: one, two, three, four, and five.