http://trac.webkit.org/changeset/42301 added code to OSX WebCore to use a plug-in to open standalone media documents with non-linear media. An error in the patch makes it fall back to the plug-in whenever QTKit returns QTMovieLoadStateLoading (1000), which happens frequently with files that are not saved fast-start, or with files served over a slow connection or from a slow server.
Created attachment 29428 [details] proposed patch
http://trac.webkit.org/changeset/42475