This is a rule of thumb. You should always check your assumptions about code being correct or not. However, if somebody and their code have proven themselves unreliable, it is a useful optimization to first consider they are wrong, instead of first considering they are right (humility.)

Whenever reviewing things to identify potential issues. If you assume (even justifiably) competence, you'll miss the real issues that exist.

OTOH, when proposing to make changes of your own, humility is definitely called for.