Go to a video on YouTube; it starts right away. Go to a musician's page on Myspace; the music starts right away. Everyone knows how annoying this is. What I've just figured out is the common principle in the annoyance.
There is an entire environment on your computer, and both these errors - and all the many variations - come from a single failure in planning user experience. Nobody's app or site will ever be the only thing running on anyone's machine under normal circumstances.
You can avoid this entire category of errors by remembering this simple fact.