Like all products, GM tested everything they could think of. This particular condition arose in a somewhat peculiar set of circumstances: the truck had to be heavily loaded, going up a steep hill, in first gear, and running at the minimum speed the engine could run (so that the idle speed governor was controlling the engine speed). Only under those conditions did the instability exhibit itself. It probably didn't help that the control code was buried in the middle of a bunch of microprocessor code that handled all of the engine control logic, and testing these sorts of program is notoriously difficult (think about Microsoft Word).