Post any defects you find in the released or beta versions of the ImageMagick software here. Include the ImageMagick version, OS, and any command-line required to reproduce the problem. Got a patch for a bug? Post it here.
I have a lot of pairs of images and I am using 'montage' to combine them side-by-side. Most of these work fine, but a few come out mangled. For example:
Your command and images work just fine for me on IM 6.8.2.2 Q16 Mac OSX Snow Leopard.
I would guess that you might have an issue with an overly full tmp directory for where IM is keeping its intermediate images or something stopped your processing prematurely. Are you on a shared server?
Otherwise, I would suggest that you upgrade, since your version if IM is over 100 version old.
It looks like the bug is fixed in newer versions of ImageMagick - I built 6.8.2-10 Q16 from source and the problem does not seem to occur anymore. I'll bump this topic if I run into it again. Thanks!
edit: for completeness: my /tmp is not full, and this is not a shared server.