As for someone forgetting, I can see that happening. Some people do have genuinely bad memories, and are not being manipulative or evil or uncaring. I think it depends on a few things, like is this person forgetful about everything, or do they just conveniently 'forget' things they don't want to deal with? Also, is it an adult allergy and the person never lived at home while dealing with it? And, is there a big long list of allergies and the parent forgot one?
This. My sister has had known allergies/reactions for years with pork and mushrooms. I love pork and mushrooms. I'm perfectly respectful to my sister and while I enjoy bacon or ham from time to time (digestive problems for her), our apartment is a mushroom free zone (wheezing/hives/facial swelling/throat closing for her).
My dad is on the board of directors for a large organization. He was the SINGLE person in charge of food choices at one of their fundraiser dinners for a year I couldn't attend. There was bacon in the green beans, mushroom sauce on the vegetarian dish, pork as the main meat, and mushrooms in the salad. The only thing my sister could eat was the bread and dessert. My dad didn't remember that mushrooms caused my sister's throat to close or that pork made her absolutely ill. He also tried to send me some clam chowder one time, which makes me violently ill, it has since the 90s. Not a new reaction. But he just doesn't remember it.
Having said that, OP, you are NOT in the wrong. At the fundraiser dinner, the menu was set long before my sister called my dad out on it, but when she mentioned to him that I was allergic to clams, he sent home another soup for me. If my mom was cooking dinner and sometime before the day of the event was told about someone's food allergies (especially ones she should have known about beforehand, being her own children's allergies) the menu would absolutely change so everyone could enjoy it. Your mom is in the wrong, hands down.