It really, really depends on the use-case. You certainly don't want bugs in your MMU, process scheduler, sensors, security libraries, etc. But a piece of line-

It really, really depends on the use-case. You certainly don't want bugs in your MMU, process scheduler, sensors, security libraries, etc. But a piece of line-of-business software that sends out the wrong email once because someone made a bug in a batch script? Sure. Not the end of the world. Users will tolerate some level of errors. Even if that level is sometimes expected to be 0.