A couple of things can happen when a part is machined with a CNC. First, as cutters are sharpened they get smaller, and the machine is programmed to compensate for that. If a cutter that has been sharpened many times is replaced with a new one, or vise versa as might have happened in this case, a few parts can be machined before the error is caught. Not all parts are gaged usually, but are gaged in some frequency that is related to the "risk" of that operation. Let's say that 1 in 25 parts gets gaged. If one is found bad, the other 24 should be rechecked too, but sometimes that doesn't happen, or one gets missed. Usually though the first part after a cutter change is always checked......... unless the cutter change came at a shift change, or operator change, etc., and then it could be missed. The other thing that can happen is that the part itself might not have been fully seated in the holding fixture, and that would cause the cuts to be wrong....... Computers don't fix everything.
Edit: Oops, didn't mean to respond to such an old post........ found it in a search for something else...... sorry.