My wife was part of a group that ran one for a bit at a science fiction convention in the 80s. The concept was for "those men and women in fandom who did not get to attend their prom as teens." In many cases, it was because they were unpopular nerds as teens, in military families, health issues, and so on. "Let's run a prom like we wanted to have." So instead of some boring "popular kid celebration" most proms were, it was just a bunch of nerdy fun men and women who dressed up in costumes, drank, and hung out.
The events DO collide, annihilating each other without collateral damage. That's the intention, at least.
A badly calculated anti-prom usually only causes minor irregularities though, such as people switching to preferring tea over coffee and vice versa for a few months.
Starbucks hate it, but Pickwick and Yogi Tea love it.