Same fallacy as in:
Infinity x Zero = ???

:)

http://answers.yahoo.com/question/index?qid=20080731142026AAG8eK2

x - y = 0
therefore, dividing both side by (x-y) (crossing them out) is illegal operation in mathematics.

Fallacies based on division by zero
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It is possible to disguise a special case of division by zero in an algebraic argument, leading to spurious proofs that 2 = 1 such as the following:

With the following assumptions:
0x1 = 0
0x2 = 0

The following must be true:
0x1 = 0x2

Dividing by zero gives:
0/0 x1 = 0/0 x2

Simplified, yields:
1 = 2

The fallacy is the implicit assumption that dividing by 0 is a legitimate operation with
0 / 0 = 1.

Although most people would probably recognize the above "proof" as fallacious, the same argument can be presented in a way that makes it harder to spot the error. For example, if 1 is denoted by x, 0 can be hidden behind x − x and 2 behind x + x. The above mentioned proof can then be displayed as follows:

(x-x)x = x^2-x^2 = 0
(x-x)(x+x) = x^2-x^2 = 0

hence:
(x-x)x = (x-x)(x+x)

Dividing by gives:
x = x + x

and dividing by gives:
1 = 2

The "proof" above requires the use of the distributive law. However, this requirement introduces an asymmetry between the two operations in that multiplication distributes over addition, but not the other way around. Thus, the multiplicative identity element, 1, has an additive inverse, -1, but the additive identity element, 0, does not have a multiplicative inverse.

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