My first thought is to go with Crisis on Infinite Earths. It promised so much but it was so confusing in the end. Yes, one outcome was JB on Superman. We got to see all of his reimaginings of Superman's mythos. That was well executed and necessary. The rest of it I don't think was that necessary.

Jason Czeskleba wrote: In 1971 comics were 22 pages of story for 15 cents.  Inflation was necessitating a price increase, and both DC and Marvel decided to try increasing their page count along with the price.  In the summer of that year, they both increased their books to 38 pages of story for 25 cents.

Technically this looks like the biggest mistake. If I were writing an official history of comic books for some reason I would definitely want to consider this. Also, the 90s distributor debacle which Matt Hawes has been discussing would have to go up there for consideration.

Emery Calame wrote: Using Vertigo to adultize certain of  their mainline superhero books? Including Kid Eternity? KID FREAKIN' ETERNITY?

The idea that the alternative crowd might find Kid Eternity appealing gets an honorable mention.