Repro steps: 1. Start webkit 2. Go to http://youtube.com (or any site with flash, I think) 3. Turn on "Private Browsing" crash. It happens on this line "[_pluginPackage.get() pluginFuncs]->setvalue(plugin, NPNVprivateModeBool, &value);" of privateBrowsingModeDidChange in file WebNetscapePluginView.mm
Created attachment 42152 [details] Crash info.
It looks like this was introduced with r49512.
*** Bug 30525 has been marked as a duplicate of this bug. ***
<rdar://problem/7350269>
http://trac.webkit.org/changeset/50308