Just guessing here, but some video codecs don't work well at a colour-depth of 16 bits (DivX, for instance). Then again, some were designed to work only at 16 bits or lower (Microsoft Video 1, for instance), or with a YUV colourspace instead of Poser's RGB. And still others have resolution limitations (width and height must be evenly divisible by 2, etc.).

The bottom line is that all sorts of things can cause problems when you're rendering to .avi, and should Poser crash midway through the operation, the entire .avi is corrupted, whether it was creating the output correctly or not. For these and other reasons, I moved on to image-sequence output, and create my .avi files from the images through third-party utilities. It gives me far more control over each stage of the process.

And I'm glad that what little information I provided was useful.