2. Professor Charles Xavier (X-Men: The Last Stand) Mishandling The Whole Phoenix Situation
When it came to the Phoenix, Professor X dropped the ball spectacularly. Visiting Jean Grey as a little girl, Charles Xavier decides the best way to control the powerful force within her is to telepathically repress the 'Phoenix'. In his attempt to keep that power locked up and then not reveal the nature of its destructive force to Jean as an adult, he spectacularly drops the ball, leading to the death of many, himself included. In sacrificing herself to save her fellow X Men at the end of the last film, Jean Grey unwittingly unleashes the Phoenix. She begins her path to darkness by obliterating Jean's love Scott Summers (and in doing so unceremoniously dumping Cyclops from the X Men film franchise) and turns on Charles himself when he tries to control the Phoenix. His 'death' sparks the way from Magneto to induct the Phoenix into the Brotherhood of Mutants, turning one of the X Men's great allies into a powerful enemy. An epic fail on Charles Xavier's part. And okay, Professor X may have lived to fight another die (perhaps we'll find out exactly how he's back in his original form in X Men: Days Of Future Past), but his method of control came at a great cost. Wolverine had to kill Jean to stop the Phoenix and that's something he carries with him going into his second solo movie. So between getting himself killed and mentally breaking an indestructible murder machine, you can argue that plenty of balls were dropped.