One classic example of this, though now resolved, is Euler's polyhedron formula $V - E + F = 2$. The formula initially was asserted without qualification, and gradually people began to point out cautiously (I suppose in deference to Euler) that there were "exceptions" to the theorem. The history of this formula is explored in Imre Lakatos's lovely book Proofs and refutations, written in the form of a drama, but with copious historical references.

This was a fertile error in the sense given above by Peter Heinig.