Ingredient combinations are rarely a problem. Mishandling ingredients could in some cases cause trouble:
-Putting anything that isn't supposed to be consumed raw, due to being unsanitary when raw (mostly animal derived ingredients - eg non-sushi grade fish, pork, chicken), on the plate raw.
-Putting anything that is toxic when raw or incorrectly prepared on the plate raw (mostly applies to plant matter - eg some mushrooms, cassava, taro, kidney beans, candlenuts, or unusually huge amounts of vegetables containing solanine, phasin or oxalic acid)
-Using parts of a plant or animal, bought un-pared, that aren't supposed to be eaten (eg the wrong parts of rhubarb).
-Using unfit amounts of (25%) distilled vinegar, pearl ash, lye or other ingredients with extreme pH values (it is likely that a dangerous dish would also be unpalatable).
-Using far too high amounts of certain spices - half a can of ground nutmeg or ghost pepper would probably not do you much good (but would make the dish obviously unpalatable).