IE11 introduced background navigation caching, which was meant to freeze and hide a page when the user navigated away. I was lucky to find that this freeze didn’t apply to plugins running inside iframes — a Flash file embedded in an iframe would keep running indefinitely, even as the user browsed to completely different sites.
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head><title>Resident_Plugins_BackgroundNavigationCaching</title>
</head>
<body>
<iframe src="loop.swf" width="140" height="100"></iframe>
</body>
</html>A Flash file loaded in a sandboxed iframe continued executing in the background after the user navigated away, with the audio playing audibly on top of whatever site they visited next. The background caching feature that was supposed to optimize forward/back navigation inadvertently kept the plugin context alive. Any script or network activity the Flash file was performing would continue as well, not just audio playback.
Found during my years at Microsoft (2006–2014). These bugs were patched long ago — shared here as a historical record for learning purposes.