What is one sentence that can be spoken grammatically correctly, but can never be written grammatically correctly

I don't think that a sentence like that exists. Written English is modelled after spoken English, so you can't have any differences.

the unsaturated fatty acid is a fat consist of one or more double bond in the fatty acid chain. if it contains one double bond then called as monounsaturated fatty acid however if, it contains more than one bonds called as a polyunsaturated fatty acid. true fatty acids are called unsaturated fatty acids as it reduces the risk of high blood cholesterol and good for health.