Has it ever happened that a theorem of the form
If $P$, then $Q$
was proven and published, perhaps with great difficulty, only for someone to realize later that the condition $P$ of the theorem is never satisfied, or, worse, that the conclusion $Q$ of the implication is false?
For example, if the Generalized Riemann Hypothesis were disproved tomorrow, I would have a large supply of examples on my hands, as so many results are conditional on GRH. But surely, this must have happened before, in the long history of mathematics?