I am assuming competence. Competent people make these mistakes.

If "assume competence" means "assume that people do not make the mistakes they are observed to make", then why write tests? Wherefore bounds checking? Pilots are competent, so pre-flight checklists are a waste of time. Your doctor's competent: why seek a second opinion? Being mindful involves compensating for these things.

It's possible that you're just that good – that you can implement a solution "as simple as not making these mistakes" –, in which case, I'd appreciate if you could write up your method and share it with us mere mortals. But could it also be possible that you are making these mistakes, and simply haven't noticed yet? How would you know if your understanding of the program didn't match the actual program, if you've only tested the region in which the behaviours of both coincide?