Using latest Windows install binary: 2.2.6
We have a scene with lots of spheres, with a plain black background. Using multithreaded processing, we see rectangular areas of the background getting filled in with a corresponding rectangular area from the scene. On a 1920x1080 resolution render on a quad-core, we see this in about 25% of images, a few with more than one corrupt area. The problem can be avoided using single threading; this is not a problem for us as we can run multiple versions of the renderer in parallel on different frames.
Our current rib files are large (around 38meg each). We will provide an example, or try to make a simpler version, if you request.