Here is a proof that 2 = 0:

assume x=2 **X = 2**

multiply by x on both sides X*X = 2X

subtract 4 from both sides X*X - 4 = 2X - 4

factorise (X+2)(X-2) = 2(X-2)

divide by (x-2) on both sides X+2 = 2

subtract 2 from both sides **X = 0**

We started with X = 2 but we have showed that X = 0 at the same time! Where does the logic fail?