Quoting Simon Willison:

Proving to yourself that the code works is your job.

A general rule for programming is that you should never trust any piece of code until you’ve seen it work with your own eye—or, even better, seen it fail and then fixed it.

Across my entire career, almost every time I’ve assumed some code works without actively executing it—some branch condition that rarely gets hit, or an error message that I don’t expect to occur—I’ve later come to regret that assumption.