See, e.g., http://tinderbox.mozilla.org/showlog.cgi?log=Mobile/1302975754.1302977256.14989.gz#err1 or any Android R1 log.
Looks to be something to do with antialiasing of the rounded corners (which probably isn't too much of a surprise, since the border-radius tests seem to be littered with blood-soaked notes screaming "the horror, the horror, exterminate Cairo!").
Created attachment 526542 [details]
Oh, that's why I hadn't clicked submit yet, hadn't attached the log.
Annotated as failing in http://hg.mozilla.org/mozilla-central/rev/8a2481966e27
Fixed by bug 634759