Once upon a time, back when comics were fun, Keith Giffen co-created Ambush Bug in the pages of DC Comics Presents. He showed up first in issue 52 and then again in issue 59, the infamous Superman/Legion of Substitute Heroes teamup. If you've never read it, stop what you're doing right now and go to your local comic shop and buy it, then read it. Or read it in the store, if they don't mind.
OK, so now you've read one of the best Legion stories ever. Levitz and Giffen at their peak. And really funny - who knew? That's the issue in which the Subs became a joke, by the way, which wasn't really fair to them, but they're fictional characters so they didn't protest too much.
Ambush Bug appeared in several of his own miniseries and specials in the 1980s, and the Legion had some odd cameos (because of Giffen).
For example, who can forget Julie Lad, from "Ambush Bug Nothing Special" (1992) (as seen here with R.J. Brande and Cosmic Boy)?
Over at the Legion Wiki, you can find a list of the Ambush Bug stories that the Legion has been somehow involved in, whether it's Bug talking to Paul Levitz, the Super-Pets, or the SW6 Legionnaires.
But now, Ambush Bug is coming back this summer, where Giffen promises to poke fun at all of DC's crossovers. I think somewhere in here we'll find that Bug is responsible for all the Legion reboots. It may even guest star Dark Proty, who makes his debut in an upcoming DC Nation page (sneak peek at the page on DC Nation's Myspace page).