What do you do when your kids throws a fit in public?

Last night at my work there was a lady checking out that has a kid that is about 3 or 4 years old. The lady is a regular shopper and always has this child in tow. This poor child screams at the top of her lungs and always looks a little wild eyed. She screams so loudly you can hear her from the back of the store. Last night was the worst I’ve ever heard this child screaming. It literally sounded like someone was killing her. The mother had a huge order and kept telling the cashier and people around her that the child was autistic. I felt sorry for the mother, but honestly, she was disturbing everyone, shoppers and workers. I understand now that the child has a problem and probably can’t help herself, before I always just wished that lady would spank that kid and make her hush. My nerves were so frazzled that I could barely function. I was dropping things and shaky. Listening to 15 minutes of uncontrolled screaming is nearly unbearable. It’s not just a kid throwing a fit, it’s horrible.

I personally would have just left. I don’t think I need anything bad enough to stand there and listen to that and subject everyone else to it also. There were many times when my kids were young I would just leave the store or the pizza parlor so as to not irritate everyone in earshot. How do/did you handle this type of situation?