There are several separate problems currently being had with video artifacting being experienced on the Boxee Box and in the Boxee software. This post is designed to help people spot when it is being caused by incompatibilities between older versions of h.264 encoders and current hardware acceleration methods. In particular, videos encoded with older versions of x264 (roughly two years old or more) will often display this sort of artifacting.
The artifacts being discussed here will manifest on any known platform using hardware-accelerated AVC (h.264) playback, but they will often appear more or less severe on different devices using different decoding engines.
Here is a recent example from the forums. This encode of The Matrix was done more than three years ago. This is a picture taken of the artifacts as they appear on the Boxee Box:
And here is how they appear on the exact same video file when playing through VLC (with GPU acceleration enabled) under Windows 7 with an Nvidia card:
The artifact remains regardless of the hardware decoder being used. The only circumstance under which it will not manifest is when the video is being decoded purely in software.
Remember that hardware acceleration of video files in consumer devices is still relatively new. Even a couple of years ago, it was bleeding edge.
Videos encoded with any version of x264 from roughly the past two years don't seem to exhibit this problem. So the key to avoiding this is to make sure your encoding software is up to date. Advancements and refinements to encoders like x264 are being made all of the time.