It's human nature to be confident in your knowledge. We have an innate need to know what is, what was, and what will be. Uncertainty creates anxiety and discomfort.

That's why discovery is so thrilling. But it's also why people cling to the ideas we make up to explain things. It's why our memories feel like facts, when we know from experience that they can be flawed or even manipulated.

Everyone reacts differently to being wrong, and will react differently to being challenged on what they believe to be true. There is no line, because every situation, every person, is different.

In those situations, I try to take a moment and empathize with the person who is wrong, and consider how they feel about what they believe. Is it an emotional topic? Something they are passionate about? Is it an important thing to get right, or is it an inconsequential bit of trivia?

What sort of person are they? Are they concerned with their appearance of authority or competence? Will they think you are helping or attacking them?

Usually when you factor in the nuance, the answer is obvious. Sometimes it isn't, though, and you just have to make a judgement call and see how it goes. You might get it wrong, and that's OK.