My favourite is "prove that the square root of 4 is irrational". It produces a "proof"... and if further interrogated admits that 2 squared is 4, but insists t

My favourite is "prove that the square root of 4 is irrational". It produces a "proof"... and if further interrogated admits that 2 squared is 4, but insists that the square root of 4 is nonetheless an irrational number approximately equal to 2.00000.