@sealand: common mode errors
"the [toolchain] had made an error, but the simulator had the exact same flaw that cancelled the error out in the simulation."
For the same kind of reason it can help to have very independent people writing the code and writing the test data, too, at least for part of the exercise. Doesn't stop them both making complementary errors - been there seen that got the scars.
Biting the hand that feeds IT
