Yes, despite Marvel or DC ever making it official, there’s one villain thatSupermanand theFantastic Fourhappen to share. Both the Man of Steel and Marvel’s First Family have interesting rogues' galleries and, as it happens, one particular villain from each of their franchises is actually the same person.
Superman fans knowall about Mr. Mxyzptlk, the Fifth-Dimensional imp and the godlike power he possesses. But as revealed at the end of one shocking story, Mxyzptlk is alsothe Fantastic Four’s attention-seeking nuisance, Impossible Man. However, bothMarvel and DC Comicshave (so far) been reluctant to follow up on that surprise reveal.

Superman and Fantastic Four are Both Annoyed By the Same Exact Character
Mr. Mxyzptlk and Impossible Man are One and the same
In the wake ofCrisis on Infinite Earths, crossovers between DC Comics and Marvel Comics were becoming more commonplace. But aside from licensed crossovers likeThe Uncanny X-Men and The New Teen TitansorSuperman vs. The Amazing Spider-Man: The Battle of the Century, Marvel and DC had taken to including cameos of one another’s characters.
Case in point, inSuperman #50by Jerry Ordway, Curt Swan, John Byrne, Kerry Gammill, and Dan Jurgens, Superman finds himself dealing with a threat that goes beyond his world, the inter-dimensional imp, Mr. Mxyzptlk. Mxyzptlk had given Lexa kryptonite variant that robbed Superman of his powersand the Man of Steel was none too happy with the menace.

Mxyzptlk realizes he’s forgotten to change into a form more suitable for this new world and turns into Impossible Man…
Superman got his powers back and battled with Mxyzptlk, sending him to another dimension. Mxyzptlk realizes he’s forgotten to change into a form more suitable for this new world and turns into Impossible Man (albeit with the color scheme reversed). Impossible Man begins annoying the Fantastic Four, who ignore him until he disappears once more.

Why Marvel and DC Have Never Followed Up on This Impossible Connection
For Over 20 Years, There’s Been No DC and Marvel Collaboration
While intended to be an Easter egg and not something to be taken seriously, having Mxyzptlk and Impossible Man be the same person makes a lot of sense. They’re both incredibly powerful, and mischievous, pranksters who prefer to act out against their enemies instead of using their power for actual evil.
But as much sense as it might make for Impossible Man and Mxyzptlk to be the same person, neither of the Big Two publishers followed up on that plot point. It’s understandable, as both publishers owned their respective characters and the idea ofcanonizing their connection would have been a legal nightmare.
…no story elaborated on the connection between Mxyzptlk and Impossible Man…
But interestingly enough, Marvel and DC Comics had plenty of times to at least play with the idea.Superman #50was published in 1990 and the ‘90s were the peak time for DC and Marvel’s crossovers. But no story elaborated on the connection between Mxyzptlk and Impossible Man, not even Amalgam Comics (which would have been the perfect opportunity).
Not to mention, DC and Marvel stopped doing crossovers altogether in the early 2000s. For over two decades, the Big Two hardly touched on each other’s worlds (beyond coy references and playful ribs and jests, of course). Sadly, the connection between Mr. Mxyzptlk and Impossible Man becamemore of a footnote in comic history than anything else.
Now’s the Best Time to Bring This Marvel/DC Connection Back
With Crossovers on the Horizon, There’s Never Been a Better Opportunity
It’s an interesting time to be a Marvel and DC fan. After all, fans everywhere this summer have their eyes on two movies:SupermanandFantastic Four: First Steps. Not to mention, DC and Marvel are going to be crossing over later this year inBatman/Deadpool(with more possibly on the way).
It almost seems like kismet that DC and Marvel are collaborating again, especially right when hype is through the roof for Superman and Fantastic Four. It seems like now would be the best time for both publishers to revisit this plot point. After all, who wouldn’t want to see Mxyzptlk/Impossible Mantake on the Fantastic Four and Superman at once?
…neither side will likely want to commit to any long-term change.
Again, DC and Marvel own their respective characters and beyond a one-off story, and neither side will likely want to commit to any long-term change. But if DC and Marvel were to re-examine the Impossible Man/Mxyzptlk connection, now would be the best time to do itwith fans more interested in the Fantastic Four and Superman than ever.
At the end of the day, the Impossible Man/Mxyzptlk connection is a fun Easter egg for both Marvel and DC fans to enjoy. Readers may never see this plot point ever addressed again, but whenSupermanandFantastic Four: First Stepshit theaters, it’s fun to keep in mind the secret villain both franchises share.