With the new stochastic reflection model of release 0.46 light rays are virtually reflected infinite times instead of breaking up at a certain level of recursion (SpecLightDepth in release 0.45).
Therefore you shouldn't use materials with too high reflection values.
This wont cause an endless loop and even don't make the renderer very much slower, but the results will be looking very strange and unnatural.
Note, that in nature even white paper has only a reflection coefficient of 0.75, a white wall 0.85, and a glass/silver mirror reflects only 90-94% of the incoming light.
Especially after importing scenes and materials from other rendering packages, this may be worth a short look as RGB reflection coefficients of up to 1.0 are not rare there.

Don't place your lightsource too far away from the visible scene!
This will make the lighting pass terribly slow, as light is distributed equally in all directions, because there is no importance sampling implemented yet.
The more distant the light is away from the scene, the fewer photons hit the visible part of the scene and shooting 99% of the lightrays into empty space makes the lighting very inefficient and time-consuming.