Blender 2.63, 2.64 test builds and SVN head( in the freestyle branch)
Linux/Ubuntu 12.04 64 bits
Nvidia

I've noticed a really annoying bug related with zbuffer, however I have not been able to isolate it.

When you are using a few layers with zbuffer and/or z combine composition node the composition results(renders) are not stable.
This behavior can happen during the render of an animation, or if you simply press F12 a few times.
However those results are not purely random. In the attached file I noticed two possible results, but in other production files I noticed up to 4 (maybe 6, but not sure) different results. Maybe the number of different results is related with the number of layers used in the composition.

I think the problem is in the Z output of the RenderLayer nodes. But I'm just guessing.

Attached the blend file and 2 possible results I got pressing F12 a few times.
Steps:
Open the file, press F12 wait for the render. Press again F12 and wait for the render. Repeat until you see a different render. Patience, sometimes I had to repeat the render a lot of times.