[PatrickHenry]

Or those who think that obviously Sin(2*x)=2*Sin(x).

If you knew how long it's been since I thought about factoring, you'd be more gentle in posting such stuff to me. But even I can spot the proplem with that one.

An example I'd rather use would be those who, upon being told if P then Q, think that obviously -P therefore -Q.

[Doctor Stochastic]

Or just as common: If P then Q, Q therefore P.