The following layout test is very flaky on Mountain Lion: webgl/1.0.2/conformance/textures/tex-image-and-sub-image-2d-with-video-rgb565.html It seems to have become much worse a couple weeks ago: http://webkit-test-results.appspot.com/dashboards/flakiness_dashboard.html#showAllRuns=true&tests=webgl%2F1.0.2%2Fconformance%2Ftextures%2Ftex-image-and-sub-image-2d-with-video-rgb565.html
Marked is as flaky in <http://trac.webkit.org/r161489>.