Ever since OS X Mavericks came out, I’ve been having a hard time connecting my Mac to external monitors and projectors. In previous versions there were dozens of output resolution options in System Preferences, but in Mavericks there are only two or three by default. This can cause major problems if you are connecting to a video system that has a video switcher or Crestron control system in between the computer and the projector, because the OS sees the intermediary device and may think that the the system is a native 1024×768 resolution system, when in reality it is a 1280×768 system. Now you are projecting with black columns on either side of your image: not professional.
The solution? The option button. Connect your machine to the projection system and go to System Preferences > Displays and then ⌥ Scaled. The original, exhaustive list of resolutions will appear as well as the ability to select refresh rates other than 60Hz.