The reftest css-gradients/aja-linear-6a.html fails on OSX and only there. Inspection of the images shows 13 pixels, in a diagonal line within the solid yellow region at the upper left hand corner, that are #ffff00 in the reference but #fefe00 in the test. This is a divergence between a canvas gradient and a CSS gradient, so it might be our bug, but its only showing on OSX makes me think it's more likely to be a Quartz issue. Attached is a tarball containing the parsed-out reftest images and an HTML wrapper that can be used to compare them easily.
Pushed http://hg.mozilla.org/mozilla-central/rev/3190bbe377cd to mark it as only failing on 10.5, since it passes on 10.6. Dunno whether that actually means we should resolve this NOTOURBUG or not, though.
FWIW, I'm on 10.6, and it fails for me consistently as well.