compositing/transitions/transform-on-large-layer.html has been failing intermittently on Mac: http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20-%20webkit.org&tests=compositing%2Ftransitions%2Ftransform-on-large-layer.html
What I'm seeing is not so much that the test is flaky, but that it's failing all the time. run-webkit-tests just fails to properly report that. On Mac WebKit1, the test always times out, because resizing the window is not implemented. We store the new size, and report it when asked, but don't actually resize the window, so there is no resize event. This can be easily fixed by adding [sender setFrame:frame]; to DumpRenderTree's UIDelegate.mm. Mac WebKit2 also always times out, although resizing is implemented there. I could not figure out why in the time I had for this.
See also bug 96642.