CAS simplifying non-zero expressions to 0

[parisse]

When I learned physics, I was always told not to use numbers in equations, but always symbols, and replace them only at the end (when all computations are done) by their numeric values. If you follow this rule, you will never have these kind of problems with CAS instructions like simplify, expand, etc.
A CAS can not work properly with approx data, it has to replace them by exact data, and it can not guess if 1e-34 or any other value is near 0 because of the dimension of the quantity (I mean unit choice, since dimensionless number are rarely close to 0) and should not be replaced by 0 (but should be replaced by a nonzero symbolic constant), or if it's a rounding error and should be replaced by 0.