Had a little problem when trying to use an external monitor. Somehow I got in to mirror mode, and when I did I lost picture on both the laptop and the external monitor. Found a great fix.
It’s a little app you drag onto your control bar. Basically it allows you to switch in and out of mirror mode, without hunting through menus. I use it by placing my mouse over the button, then plugging in the second monitor. If things go goofy, I just click the trackpad button and it tends to right itself. Nice.