Dr Datacharmer's Brain Training
The classic proof that 2 = 1 runs thus. First, let x = y = 1. Then:
x = y
x2 = xy
x2 - y2 = xy - y2
(x + y)(x - y) = y(x - y)
x + y = y
2 = 1
NB: x2 signifies x squared.
Spoilers at overcoming bias, via Chris Dillow.
(x-y) = 0. Therefore you divided through by 0- which isn't allowed, you wicked beast.
Old dividing by zero, the number of times I fell for that one while attempting game theory. Damn zero