Men feel their masculinity is threatened when they are told they are less assertive, dominant, or 'masculine' than others. They may also experience these reactions when they find themselves to be subordinate to a woman who clearly takes the lead or when they are expected to perform tasks considered 'unmanly.'
Mistakes are almighty: you can't ever guarantee that the next moment will host no manifestation of a mistake. According to evolution theory, the diversity of life on Earth entirely emerges from copying mistakes of DNA polymerase.