It looks like the culprit might be http://trac.webkit.org/changeset/83351
Hrm, this failure appears to have disappeared by r83355. I have yet to see evidence that it was flaking ...
It is flakey and should be skipped: http://build.webkit.org/results/SnowLeopard%20Intel%20Release%20(WebKit2%20Tests)/r84295%20(10862)/results.html
Added to the mac-wk2 Skipped list in http://trac.webkit.org/changeset/84369