-Turning Austin heel in 2001. Austin was still a red-hot babyface and turning him heel at the time (especially in his own home state of Texas) wasn't the brightest of moves. It also contributed to a drop-off (not a massive one, but a noticeable one) in business at the time, if you really want objective proof that it wasn't the most splendid of ideas. The turn itself was Austin's idea because he felt he was getting stale after returning in late 2000, but even he looks back on it now and regrets that it happened.
-The Nexus losing at Summerslam 2010. This would be akin to if The Alliance had lost at the 2001 Invasion PPV. This big, bad stable that just ran through and destroyed several superstars gets defeated in their first real chance to prove they're a threat? That's pretty much the antithesis on how to get a dominant heel stable over.
-Speaking of the Invasion angle... um, yeah, where to even begin. Not acquiring more WCW talent (Sting, Goldberg, Mysterio, Flair, Bischoff, etc... although I realize a couple of these were out of their hands), making Shane the owner of WCW and (especially) Stephanie the owner of ECW and turning the whole angle into a McMahon Family storyline, having Team WWF destroy most of the Alliance on a constant basis, having Austin constantly berate his own guys for losing a match, having Austin as the leader of Team WCW, etc. Look no further than Survivor Series, where the angle met it's conclusion, for the kind of logic this whole thing encompassed - The Rock, leader of Team WWF, is the WCW World Champion, and Steve Austin, leader of Team WCW, is the WWF Champion.
-The Hogan/Sting debacle at Starrcade 1997. Nick Patrick was supposed to give Sting a fast-count and make it appear that he had been screwed in Hogan's favor, but instead, he fucked up and gave a regular three count instead (or rather, as according to some, was bribed by Hogan to make a normal count) and it just made it appear as if Sting had been soundly defeated by Hogan... which is among the biggest fuck-ups in history. Fuck all the attempted screw jobs, just have Sting go in and defeat Hogan to win the championship to give WCW their big moment. No shenanigans needed.
-Not following up on the Fingerpoke Of Doom. I thought, unlike most, that this was a great angle, and it would have been perfect if only they had followed through with Goldberg tearing through the NWO and eventually making his way back up to Hogan and reclaiming the WCW World Heavyweight Championship. Unfortunately, it never happened and they then proceeded to do jack shit with Goldberg for the remainder of his WCW career.