I vagely remember an issue of either Batman or Detective that was set in Rutland, possibly written by Denny O’Neil, definitely drawn by Neal Adams and Dick Giordano and had a panel featuring a drunken guest dressed like Thor talking to Harlan Ellison. The ersatz Thor says to Harlan, “You write comic books? What a dumb way to make a living.” To which Harlan replies, “You’re standing on my foot.” (probably paraphrased)
That’s all I remember from that issue, I don’t even remember the plot, but I know it was either Batman or Detective.
I have a better recall of JLA #145. I recall one panel with the JLA trapped on a roller coaster that was copy of a panel drawn earlier for MAD by Don Martin.
There were strong hints that the Impossible Man and Mr. Mxyzptlk were actually the same person. Best example I can remember of this was during the “Krisis of the Krimson Kryptonite” (yes, they spelled it that way) storyline in Superman a few years ago. Mxy is in a rather impish form on an alternate Earth, battling a foursome: 3 people in blue tights and a large monster, who we can’t see to well because he’s covered in glop. One of the people can turn into flame, and another stretches. We never see their faces…but c’mon…
(This has pretty much been overrided by Superman/Silver Surfer, in which Mxy and Impy had a hysterical battle.)
I just remembered two more…A Superman vs. The Terminator book collection I saw in the bookstore, once. (Even featuring a jaunt to the future showing an aged Superman aiding the human resistance led by John Connor. Which I found somewhat charming, for some reason)
The other one, though not an “Official” crossover (Perhaps more of a parody, or an Homage), was an episode of Batman Beyond, where a group of mutated scientists become am openly superheroic group calling itself “The Trio,” based out of Gotham City. They all (Well, most of them) wore blue uniforms with the team’s “3” logo on the front, and were headquartered in a “Trio Tower.” The husband and wife pair on the team even featured a non-corporial woman.
Of course, this being an unofficial crossover set in a Cyberpunk series, they were all killed off horribly by the end of the episode.