So this happened a few years back when I was working at your favorite, now defunct toy store.
For some background, the store I worked in was very old, no air conditioning and the air flow was terrible. This specific day was a very humid and hot day (New England summers are HUMID) and I had been having issues with my asthma on and off.
I had been working at the electronics desk, grabbing video games and just working as a general checkout. I grabbed some games from the lockup for a customer and headed back to my checkout desk. Two people were already waiting in line. I did my usual spiel. apologizing for the wait etc. Customer is visibly agitated already so I can tell this will be fun. She's pulling a bunch of stuff out of her cart, saying she no longer wants it. That's cool, at least she didn't shove it on a shelf somewhere. As I'm turning around to place her item she doesn't want on the counter, I can feel an asthma attack coming. I do that thing when you feel like you can't breathe so you try to inhale really hard. She assumed I was sighing at her for not wanting her items (for real, don't care, I'm not closing, not my problem).
This began the biggest freak out of the day. This woman starts yelling at me for being rude, saying it's my job to take her things she doesn't want and to be polite. As she's yelling I'm now start to stress out, making my asthma worse. As she's still yelling at me, I sit down. That was apparently the wrong move. She decides that it's the final straw and saying "I'm not buying any of this, you need to learn how to treat customers!".
She did call corporate, I was talked to by my manager about it and explained the whole thing (she knew about my asthma), which was on video. My manager actually apologized for the customer saying "I never knew an asthma attack is a personal attack on a customer." I will never forget that line or that manager. She was amazing.