That's a HUGE problem. This is an absolutely fucking nightmare. It's arguably the hardest thing to possibly debug, because you're not informed it's a bug. No amount of error handling will solve that, which you've pointed out as "the biggest problem".

> If someone calls me with invalid input, and it's coerced and you get invalid output

But this isn't the problem. The problem is when it gets called with invalid input and somehow gets coerced into a valid but unexpected output. Expected input -> Expected output means you get instant gratification as to whether or not it works. This dramatically reduces the number of error cases which naturally and effortlessly moves toward solving your complaint.