You had no way of knowing tell her this and apologise asap

Accidents happen......literally.

You shouldn't feel bad. It was an accident. You're not an asshole if you feel guilty and you admit to making a mistake.

That really sucks dude,I'm sure if you ever get to explain it to her she'll understand.

There's no way you could have known, don't feel so bad about it.

At least you can admit your mistake. I hope you could apologize about it

What I don't really understand is why she took offense. She was out on Halloween. She had to have known that someone would make that mistake. If it'd been me with a missing arm, I would have done as #7 suggested and use that disadvantage to make a zombie costume or something. Or at the very least if it'd been me, I would have understood that people shouldn't always assume things for what they really are, especially on Halloween. If you can't make jokes about yourself, or make light of things you've been through, it makes your life extra hard to deal with.

There was a girl at Universal the other night who'd made being missing an arm the key focus of her costume. I expect how she feels depends on the costume. If she was dressed as Jessica Rabbit maybe you were out of line. If she was a zombie then maybe not so.

Aww. Well, it was a legitimate mistake. I'm sure if you explained, she'd understand.

It's okay.. you didn't know