Overclocking your GPU is always a risky business, ekeing out a little more power from your graphics card at the expense of extra heat and stability. Even in a water-cooled desktop mega-rig it can burn out your graphics card, but in a laptop it's even more difficult to pull off without causing damage to your machine.
Nvidia's decided it's a risk not worth taking with its 900M series of laptop GPUs. The latest driver release will disable overclocking of the chips altogether, with Nvidia claiming it was only a bug that allowed users to ever overclock the 900M series anyway:
Unfortunately GeForce notebooks were not designed to support overclocking. Overclocking is by no means a trivial feature, and depends on thoughtful design of thermal, electrical, and other considerations. By overclocking a notebook, a user risks serious damage to the system that could result in non-functional systems, reduced notebook life, or many other effects.
There was a bug introduced into our drivers which enabled some systems to overclock. This was fixed in a recent update. Our intent was not to remove features from GeForce notebooks, but rather to safeguard systems from operating outside design limits.
The comments that follow the announcement on Nvidia's announcement page are, as you'd expect, pretty frustrated. All those precious frames per second lost like...tears...in... yeah, you've heard that one before. Still, if you're intent on risking your entire system at the expense of a slightly-smoother gaming experience, you can always roll back the driver to an older, more carefree version. [Nvidia via TechRadar]