Proof of 1 = 0, use of ill-formed statements In his book "Analysis 1", Terence Tao writes:

A logical argument should not contain any ill-formed
  statements, thus for instance if an argument uses a statement such
  as x/y = z, it needs to first ensure that y is not equal to zero.
  Many purported proofs of “0=1” or other false statements rely on
  overlooking this “statements must be well-formed” criterion.

Can you give an example of such a proof of "0=1"?

Start with the assumption
$$x = 0$$
Divide both sides by $x$ to get
$$x/x=0/x$$
and thus
$$1=0$$
That's the general scheme. Of course it generally gets more obfuscated, for example by starting with the assumption $a+b=c$ and then later dividing both sides with $c-a-b$.
