Mathematics
In

numerical analysis, cancellation is a bad thing. When small

floating point quantities are added to large floating point quantities, large errors can result. This fact is fundamental to numerical analysis.

An example, using the

MATLAB notation:

>> x = pi;

>> y = 6.022e23;

>> x = x+y -y

x =

0

It's things like this that make it very difficult to write high quality general numerical libraries. For this reason the well-tested

NETLIB libraries are still quite used, despite being written in

fortran.