Why don't waiters and cooks listen?

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This is sort of related to a previous post, but also a bit off topic, so I thought I'd separate it. I went to a restaurant a couple of months ago with friends, I don't eat out often due to food allergies, but as we were going for burgers I felt pretty safe. I told my waiter NO mayonnaise, No cheese. He brings out my burger and there was cheese and mayo on it. I sent it back, came back out without the cheese but still with the mayo. I am EXTREMELY allergic to eggs! I tried to explain this to the waiter and he seemed to think that scraping it off the bun would do the trick. NOT!!! We walked out and haven't been back. Why don't people take food allergies seriously? Have you ever had an experience like this?