I went to a dinner party last night, and it was a lovely evening, until my friend's partner and all his friends returned from the pub. I'll re-enact the conversation for you, and would like you to bear in mind that I have never met these men before, he was a complete stranger to me and most other people there.
Man1: So what do you do then?
Me: I'm a behaviour management trainer
Man1: What does that entail?
Horribleman: (shouting) You do what?
Me: I'm a behaviour management trainer
Horribleman: What the hell is that?
Me: I train staff on how to manage challenging and aggressive behaviour displayed by individual's they support at work, people with LD and mental health needs etc.
Horribleman: You mean window-lickers?
Me: No... I mean people with learning difficulties and/or mental health issues.
Horribleman: Yeah, window-lickers, slobberers, you know, mongs?
Me: ~silence~
Horribleman: It's ok, I'm allowed to say it.
Me: Why's that then?
Horribleman: Because I am one! Hahahahahahaha ~insert disgusting (and very poor) impression here~
What. A. W*@n$£r.
What I wanted to say was "Considering I've just told you where I work, I can't believe you're stupid enough to think that I would actually find that funny. Also, my baby has down's syndrome, and you've just offended me. So I think you should f$@k off now."
But I couldn't get any words out at all, I just sat there, seething; fists clenched, going very hot, with the hairs on the back of my neck standing on end and a big lump in my throat 
I'm a bit cross with my myself now for letting it get to me in the first place, I know how horrible people can be from working where I work, but this time it just threw me 