Part of the problem was that MCAS was originally designed with very little control authority, and so wasn't considered safety-critical. However, during testing

Part of the problem was that MCAS was originally designed with very little control authority, and so wasn't considered safety-critical. However, during testing they realized they needed to up the gain, and made pretty major retuning without reexamining their safety assumptions. Plus the bug with the resets on the limiter.