What do you think about me accidentally saying a trans person looked like her other friends?
Did you look at her or did she look at you? Did you say something to her?
I just said she looks like her other friends. I couldn't see her because it's a dark bar.
In a dark area you can't always tell everyone apart clearly. It's ok to make mistakes.
What if I really said something ignorant?
While it is a little embarrassing for you and them, I am sure they'll forget pretty fast and if it was an honest mistake then you have nothing to worry about. You know that you did nothing malicious, and that is all that matters.