I have been noticing lately that sometimes Chop does not succeed in reducing numbers close to 0 to 0. Although my email program scrambled the superscripts, those '-7's down there are from the 10^-7's where I would expect (and would want) to see plain 0. Mathematica version 4.01 on a Mac. Copied as plain text from OutputForm.
This failure of Chop has ruined one of my most important algorithms, which takes the Union of a list of n-vectors. It has wrecked in turn my most important notebooks. Notice, if you will, members 5 and 6 of the list below, where the z component should have Chopped to zero, but instead is two very small numbers of opposite sign.