According to the article "Custer and Cognition" published on custerwest.org, Custer main error was... to trust his subordinates and to think that they would apply his plan.
It's clearly one of the strangest conclusion of military history. It is politically uncorrect to state that Custer could have suffered from a military betrayal, despite overwhelming evidence. So the military analysts are now condemning Custer of having followed his plan, and of having expected that his entire unit would support him.
Well, what's wrong with a commander expecting his surbordinates to obey his orders?