Matrix multiplication error

I am experiencing a strage error in matlab. I have matrix A and a vector x. I expect that if I perform the following operations, C should equal zero but it does not.

B = A*x;

C = A\B-x;

In fact C is is a vector composed of the same constant. What could possibly cause this in matlab?

[Walter Roberson]

This appear to be the same as http://www.mathworks.com/matlabcentral/answers/38567-reversible-matrix-operation