Misgendering

The term misgendering means referring to somebody as if they had another gender identity. For example, using he/him pronouns for a woman who uses she/her pronouns, or other unaccepted pronouns for a nonbinary person who uses inclusive or nounself pronouns. This is one method transphobes often use to attack transgender and nonbinary people.

Unintentional Usage
It is common for someone to assume another person's gender identity based on gender cues, such as their voice, hair length or style, and clothing.

How to deal with unintentional misgendering
A lot of the time, misgendering is an accident. Most of the time, the best thing to do is gently explain to the person that you use different pronouns, and you would like it if they respected that. Also, to avoid unintentional misgendering, if you feel comfortable when you meet someone, tell them your preferred pronouns right away, if possible.