According to [spec](http://www.w3.org/html/wg/drafts/html/master/webappapis.html#runtime-script-errors), `return true` should be used to indicate the error is handled. WebKit had it [backward](https://bugs.webkit.org/show_bug.cgi?id=67119) for some time, including the version that PhantomJS uses. I will get a `return true` [upstream in mocha](https://github.com/visionmedia/mocha/pull/868) -- meanwhile see bfa55163eac6ebeb8fd5d09f6303f158fd614fef.