A programmer, when testing, will see a text box labeled "Quantity" and enter "1". A highly experienced programmer will then do a followup test with the value "2".

A user will see a text box labeled "Quantity" and enter "~~ unicorns ROX !!! ~~". An experienced user will also try "-12 1/2".

Hopefully, a tester will be in there somewhere to alert the programmer about what the user is going to experience when they do these things.