Puzzle for October 20, 2008

This is an Algebra question. But some of you folks in lower grades will still know the answer. What's wrong with this math problem?

Start with x and y being equal	x = y
Multiply both sides by x	x^2 = xy
Subtract y^2 from both sides	X^2 - y^2 = xy - y^2
Factor	(x + y)(x - y) = y(x - y)
Simplify	x + y = y
Substitute y for x (since they're equal)	y + y = y
WHAT!?	2y = y

How could this happen? For you non-Algebra folks, what's the only value x and y could have to make the last line true?