I've added some assertions for page to process relationship, and I'm seeing them fail in Safari sometimes.
Created attachment 236683 [details]
Looks like this introduced failures in debug (failing assertion):. E.g.: http://build.webkit.org/builders/Apple%20Mavericks%20Debug%20WK2%20(Tests)/builds/6284/steps/run-api-tests/logs/stdio
Filed bug 136012 about this, will investigate.