When the Justice League and the Avengers finally crossed over in 2003 in a story that brought in every member from both teams' considerable history, it was a pretty big deal. The thing is, it wasn't the first time those two teams had brushed up against each other—that actually happened in 1972, in an unofficial crossover based around Rutland, Vermont's real-life Halloween parade.
The parade became an annual event in 1960, and thanks to the fact that organizer Tom Fagan was a huge comics fan, there were plenty of superhero-inspired costumes at the parade every year. So many, in fact, that by 1971, it had been featured in both Avengers and Batman. In 1972, though, Steve Englehart, Gerry Conway, and Len Wein plotted a story that would run in the background of three different comics—two from Marvel and one from DC—on the same night.
The story featured the creators (along with Glynis Wein, Len's wife) taking a road trip to Rutland for the parade that saw them attacked by the Juggernaut and winding up in an issue of Thor. In between, they found themselves in Justice League of America #103, possessed, like many parade goers, by Felix Faust and forced to do battle with the powers of the superheroes they were dressed as. It probably won't surprise you that, since this was a DC Comic, Batman didn't have much trouble disposing of a store-bought "Commando America" or a "bargain-basement web-slinger."