opting-0.6.4 at FreeBSD-8.1-RELEASE i386, compiled by system gcc version 4.2.1 20070719 [FreeBSD] (cflags -O2 -march=prescott)
I've run optipng -o0 on bunch of generated png images I have, and it crashed on one of these. I guess that reveals some border case bug, as there's only one failure for 1473 images which only differ in content (e.g. were generated by the same code). Image is attached.
Other optimization levels (-o1, -o2, -o3) seem to work without problems.