Out-of-process plugins in WebKit1 on SnowLeopard can't execute JavaScript during NPP_Destroy. WebKit1 has code that blocks this. See <http://trac.webkit.org/browser/trunk/Source/WebKit/mac/Plugins/Hosted/NetscapePluginInstanceProxy.h?rev=78634#L350>. In-process plugins in WebKit1, both in-process and out-of-process in WebKit2, and plugins in other browsers do not have this restriction.
<rdar://problem/9251800>
Mass closing plug-in bugs, as plug-in support has been removed from WebKit. Please comment and/or reopen if this still affects WebKit in some way.