Totally fine, it happens to everyone!
What is most important is that you do not make a big deal about it. Just apologize quickly, correct yourself, and move on.
Ex: “Oh, I’m sorry, I meant they.”
If you make it a big deal, you draw more attention onto someone who maybe does not want it. As long as you portray that you are sorry and you try harder next time, it is going to be okay. Remember; this is more for them and not you, so never make your apology about you. Always make it about the person you have wronged.
What if someone else makes a mistake?
Easy, correct them politely and quickly, do not make a big deal about it.
Ex: “Actually, Maribel uses he pronouns.”
Do not ignore a situation where people continuously use the wrong pronouns. The mark of a true ally is never giving up on the people you want to help. Plus, Transgender and gender non-conforming people tend to get tired of always correcting other people, so having a friend to help is amazing.