Most generator designers use either IEC or NEMA as guidelines; the specific choice of which is applicable is often made by the end user (or at least the one purchasing the equipment). Both of these entities specify "normal" operation as falling within a set range of voltage, frequency, and a combination of both.
Normal operation occurs when ALL of the following are true.
(0.90 PU Volts < operating line voltage < 1.10 PU volts)
(0.95 PU Hertz < operating line frequency < 1.05 PU Hertz)
(0.95 PU Combined < operating combination < 1.05 PU Combined)
0.98 PU volts and 0.98 PU Hertz = (0.98 * 0.98) = 0.96 okay.
0.98 PU volts and 0.96 PU Hertz = (0.98 * 0.96) = 0.94 NOT OKAY.
I worked in the generator field for over 35 years, 20 years as a technician and the rest as a field engineer. At one time the limits were around 70 Hertz for overspeed shutdown protection. On every generator that I had to check where they went above this speed usually this is a fast ramp up on speed the generator windings would end up being cut into pieces about 3 inches long on the bottom of the generator.
In generators the 1500 and 1800 RPM machines are most often interchangeable, 50 Hz 400V and 60 Hz 480V. So a 50 Hz 4 pole machine can be over speed quite considerably.
What can happen? It depends. Rotor wedges fail. Copper winding out hangs lift, unbalanced. Turn to turn or layer to layer faults on winding, bearing failure, damage to stator from rotor failure. The list is endless, even down to on salient pole machines the pole tips lifting and exciter components, etc. Take it apart and examine thoroughly electrically and mechanically.