I'm living that right now, except he has no MBA, is a consultant, has the dogged determination of an elderly employee on his last ever career project and used his leverage to bring in a bunch of unqualified architect yes-men.

This is easily the number one for me. The amount of damage someone like that can do (while swiftly being promoted up the ranks of course) is staggering. Whenever I identify one, I start looking for jobs.