A good mentor helps you avoid their unnecessary mistakes without stealing your necessary ones.

Some mistakes teach nothing that could have been transmitted, and a person has to make them personally to learn anything at all. Others are simply avoidable and cost time for no reason. A good mentor knows the difference and protects only against the second kind, resisting the urge to spare someone every difficulty.

Before intervening, ask whether this mistake would teach them something you cannot say in words. If it would, stay quiet and stay nearby. Save your warnings for the ones with no lesson in them.