Hmm. Einstein is perhaps most famous for "discovering" special relativity. But he neither formulated the problem, nor found the solution (I think the Lorentz transformation was already known to be the solution), but reinterpreted the solution as being real.

His "greatest error" was introducing the cosmological constant into general relativity--curiously, making a similar error to what everyone else had made when confronted with the constancy of the speed of light, which was refusing to accept that the mathematical result described reality.