Why True Things are often Humbug

Assume x = y. Then x2 = xy. Thus x2 - y2 = xy - y2. Divide both sides by x - y, arrive at x + y = y. Since x = y, we have 2y = y. Divide by y, arrive at 1=2.

We violated a side-condition, cannot divide by zero, to arrive at the wrong result. Pretty nice workable theory though.

011610: Decided I am a complete nitwit when it comes to science and just asked the experts... Waiting: http://www.physicsforums.com/showthread.php?t=369914

This turned into nagging about something I shouldn't meddle in, this is food for physicists. I'll see what gives. Something completely else, C programming.