Layouttests/media/video-source-add-src.html sets the <video> element src in the 'loadstart' event handler and sets a timer to fire in 200ms to check the element state and end the test. The test also logs a number of events that fire after 'loadstart', but on a slow (or loaded) machine the 200ms timer can trigger before all of the media events fire - causing the test to "fail" because not all events are logged.
Created attachment 35131 [details] proposed patch
http://trac.webkit.org/changeset/47518