Snails will eat almost anything but that doesn't mean you should feed them everything they will eat. The foods below are harmful to snails and if eaten my be fatal.
Food stuffs they should never have include:
Avoid anything with added salt. Salt in any form including from water softeners! Be careful when giving thing like fish flakes and tortoise food because salt is often an ingredient.
Pasta puffs up, and when a snail eats it, they puff up out of their shell as they try to digest it which is often fatal.
Same reasons as pasta.
Seaweed is grown in salt water and often has salt as a preservative.