That we think have no cost. The massive failure rate of drug trials and some famous cases of issues discovered only after wide scale deployment indicates we're not that great at knowing ahead of time.

The body is like legacy spaghetti code written by hundreds of teams of outsourced engineers. It mostly works. Just never remove any commented out lines or it may break.

While possible, there are also many bodily processes that are finely tuned through eons of evolution, and destabilizing pressure leads to disorder. Sometimes it's difficult to know which are which (or at least I don't know).

Which things?