Programming

Making a mistake once in a while on something one does all time is to be expected - even somebody with a 0.1% rate of mistakes will fuck up once in while if they do something with high enough frequency, especially if they're too time constrained to validate.

Making a mistake on something you do just once, such as setting up the process for pushing virus definition files to millions of computers in such a way that they're not checked inhouse before they go into Production, is a 100% rate of mistakes.

A rate of mistakes of 0.1% is generally not incompetence (dependes on how simple the process is and how much you're paying for that person's work), whilst a rate of 100% definitelly is.

The point being that those designing processes, who have lots of time to do it, check it and cross check it, and who generally only do it once per place they work (maybe twice), really have no excuse to fail the one thing they had to do with all the time in the World, whilst those who do the same thing again and again under strict time constraints definitelly have valid excuse to once in a blue moon make a mistake.