Mussels, in a Los Angeles restaurant with a high reputation. The chef had failed to check them well enough, and at least one of them was no good. It tainted the whole dish. Not much tastes worse than a bad mussel. I too failed, in that I was involved in conversation and didn't sniff them before eating, something you should always do. With my first bite it was all I could manage to avoid turning green and rushing from the table.
I beckoned the waiter and told him to take them away. He came back with a message from the kitchen that there was nothing wrong with them. I got myself and my guest up and walked out. That restaurant does not get my trade anymore.