I wish I'd known this a few years ago.
I had a badly broken wrist which resulted in RSD (a complication effecting nervous system). My arm and hand looked REALLY strange, was extremely painful, was immobile, and I was scared sick that it was going to be left totally crippled. A difficult and upsetting time.
I popped into supermarket to buy sandwich for lunch (I was at work that day for a few hours). The cashier looked at my odd arm/hand and said 'Is that a false arm?'. Yes, it did kinda look that way to be honest, but why ask? I shot out a scowl and an abrupt 'no'. To which she reared back in horror, and shrieked 'ugh! What's wrong with it then?' Cue me just about making it back to office and dissolving in tears.
What an interesting assumption would have been a better answer to her first question as it might have shut her down. And yes, I do now wish I'd gone back and complained about her, but at the time I just wanted to forget all about it.