Back in 1972,  Marvel & DC published a crossover that was so secret, even they didn't know they'd published it!

Rogue Marvel & DC writers Steve Englehart, Len Wein, & Gerry Conway conspired to have a crossover take place between their three magazines, Amazing Adventures #15 (featuring the X-Men's Beast and drawn by Bob Brown & Marie Severin), Justice League Of America #102 (drawn by Dick Dillin), and The Mighty Thor #207 (drawn by John Buscema)  without telling their respective companies they were doing so.

The backdrop for this storyline was Rutland, Vermont's annual Halloween festival, (a real-life event, by the way) which had many of the attendees decked out in superhero garb and Englehart, Wein, & Conway inserted themselves (and their respective wives and girlfriends) as characters who wind up as part of the adventure.  

Scooby-Doo style, the gang helps the superheroes vanquish their foes and Marvel and DC saved Halloween...without either of them knowing it.  And it remained that way for years.

Hosted by Chris Karam and Brad Page