"You're obviously incorrect, as is seen without proof. QED"
"But," say Man, "the Babel fish is a dead giveaway, isn't it? It could not have evolved by chance. It proves you exist, and so therefore, by your own arguments, you don't. QED."
"Oh dear," says God, "I hadn't thought of that" and promply vanishes in a puff of logic." -- Douglas Adams, from "A Hitchiker's Guide to the Galaxy"
Proof that x + 3 = 0 if x = -3
x + 3 = (-3) + 3
2. Modern usage expands it to include the conclusion of any proof and is often used in attempts at ironic humor.
3. Quantum electrodynamics.
2. Beckham and Ronaldo move to MLS. MLS attracts more fans and makes more money. More money attracts more quality players. More quality players make USA competitive. USA wins World Cup. Q.E.D.