To be unprepared for a completely foreseeable situation and to respond in a way as to make the situation far worse.
Named for Michigan/NFL Wide Receiver Mario Manningham.
A: "Can you believe the teams that were going to pay me millions of dollars asked about my failed drug tests?"
B: "Yeah, I can, it makes a lot of sense for them to. What did you do?"
A: "Got all offended then lied about it"
B: "Aren't they going to check up on that?"
A: "Oh shit, probably."
B: "Nice job, Manninghaming that one."