I had a professor once (for modern physics, i.e. relativity and quantum mechanics) who said that if anyone could get 100% on his test, they'd saturate the "sensor" and make the measurement less accurate; he wanted to see the bell curve centered low enough that it didn't get clipped by the maximum score. He calibrated his tests so the highest scores would typically fall in the 50s or 60s, and then applied a curve. Actually getting 100 would require knowing all the material for that course in advance and then some.

An interesting, if nerve-wracking, philosophy. :) It's the only time I've ever left a test and not known for sure what grade I'd get.