Fallacy of necessity

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A fallacy of necessity is a fallacy in the logic of a syllogism whereby a degree of unwarranted necessity is placed in the conclusion.

Example:

Bachelors are necessarily unmarried.

John is a bachelor.

John is necessarily unmarried.

This example seems watertight, but the problem lies with the necessarily in c). c) suggests that it is inconceivable for John to marry - however b) does not state this; merely that, at present, John happens to be a bachelor. For c) to hold true, both a) and b) would have to be necessarily true, but only a) is, since it is a tautology.