Take a look at the damage to this Danfoss H1B160 axial-piston hydraulic motor:
This spectacular catastrophic failure was caused by over-speeding, which, in turn, was caused by a careless operation. Let me explain:
It is very intuitive to think that over-speeding would be impossible in a hydraulic system, where the minimum displacement of the hydraulic motor is set so that even when it's hit with the maximum available flow, its speed doesn't go over its max rated rpm. But this is not true.
This is not true because almost always the load can "drive the motor back", and in certain applications/situations, store significant amounts of energy due to its "elastic" or "gravitational" properties, which can then be violently released.
The bent-axis variable displacement motors, by the way, have a much lower max rated rpm when they are at max. displacement. For example - the H1B160 can run all day long at the respectable 4250 rpm when its axis is tilted at 6º, but it shouldn't go higher than 2600 when it's at max displacement. This can make things even worse for such units when the "loads fight back".
Runaway crane loads are a good example of stored energy released in an uncontrolled fashion, but this particular over-speed was caused by my favorite - the drill string of a diamond core drilling rig!
The several-hundred-meter-long string composed of dozens of steel rods threaded one into another is essentially a huge torsion spring, and when it gets stuck at depth, the drilling head on the surface can easily wind it several full turns before stopping. As you can imagine, releasing such a "wound" motor without care results in a most violent backlash and this is exactly what destroyed this poor unit.
The operator tried to bail the stuck drilling string out by jolting the rotation control lever back and forth between forward and neutral (open-center DCV), and after several attempts, the hydraulic motor (almost new, by the way) did an especially loud "z-z-z-z-z" and then ripped four pistons from the shaft before seizing.
Moral of the story: when operating and (especially) designing your hydraulic systems, always bear in mind the fact that the load can "drive the hydraulic motor back", and if it is "elastic" or "gravitational" in nature, design and/or operational precautions must be taken to make sure the stored energy is dissipated in a controlled fashion.