Rotation of Generator in Opposite Direction

Nothing wrong electrically. But, the cooling is affected as the shaft mounted fans have curvature designed for certain direction of rotation. This will cause overheating of windings and associated damage.

The exciter brushes for the DC to the rotor are often rotationally offset on the slip rings to have a leading and trailing edge which may also have unwanted consequences to the other cooling effects.

Any ancillary items on larger machines, like lube pumps, may be direction sensitive so lubrication also becomes a problem.

Apart from mechanical effects on auxiliary items like shaft driven pumps or fans mentioned by raghun, the phase rotation will be reversed.

If you then connect to a standard system without swapping generator wires, then motors will rotate backwards, which is likely to cause trouble.

If you parallel with other generators or Grid, it is possible to get 180 degrees out of phase [ a properly connected check or auto-sync unit will not allow]. While generators are designed to survive a short circuit, such a connection causes twice the current and generator will be wrecked.

One of the possibilities is being invited to work elsewhere on the grounds of a mismatch between the required, and the demonstrated, competence.