When loading a plugin that's not "embedded", the plugin will pop out of the frame and navigate up and to the left. The problem seems to get more severe if the QWebView is contained in more QFrames.
Example would be trying to load a video from youtube.com/v/VIDEOID will cause the issue, but loading a video from youtube.com/embed/VIDEOID will not. This isn't just youtube, but I've experienced this with Unity games and PDFs.
Here's an example with a PDF: http://puu.sh/3eLHw.png