I just spent an hour or two tracking down some problems with the buildbot test images that crept in unnoticed with the pdf backend testing. I think I now fixed all the issues with the buildbot testing, which required a few changes to the MPL source and a few on the buildbot server to build the image overview pages correctly. Anyhow, the image failure overview page is back to displaying useful information at http://mpl.code.astraw.com/overview.html
All this was prompted by a buildbot failure that occurred starting at r7958 or r7959, although these changes seem unlikely to be the cause of the issue. So, I'm a little mystified as to the actual cause, but it again looks like some freetype problem. Mike, do you think it could somehow be related to your recent font work?