I developed a bad reaction (not sure it is a true allergy) to bivalves - eg mussels, scallops, oysters - in my early 20s having been fine with them before.
4-5h after eating them I get a terrible temperature, stomach cramps and then awful vomiting till my body is sure they;re all gone. No lasting effects though.
First time I thought it was just one bad one, second time I blamed the (frankly unwise) amount of alcohol consumed alongside, but when it happened after some delicious and impeccably-prepared oysters in a fabulous restaurant in France I became a bit more wary.
Then at someone's wedding I gave dh my scallops and ate the salad from underneath them... and still reacted. Was 8m pg at the time and it was not pleasant .
But I am fine with all other seafood - crabs, lobster, prawns etc - and with snails, it is just things that come in a two-part shell that get me.
One day when I can be bothered I am going to try and re-tolerise myself as it is a PITA and I miss them.