I'm experiencing a strange bug: My mid-2012 13" MBP hangs when booting whenever my monitor is attached via the Thunderbolt port. Disconnecting the monitor solves the problem every time. I'm able to get as far as displaying the Apple logo on the MBP's screen; the external monitor displays a light blue screen. It's impossible to get past this point. Connecting the monitor after booting to Mountain Lion is no problem at all. FYI, I am using a Dell 2408WFP monitor connected via DVI connected to the MBP using the Apple DVI-to-Display Port adapter.