An editor I work with likes to have his iMac in the center, his Apple Thunderbolt Display to the left, a broadcast monitor to the right, and a large consumer TV on the wall for clients to see from the couch behind him. He also doesn't like to use I/O devices from the likes of AJA, Blackmagic Design, and Matrox as he finds them fussy. He's not wrong, but supporting this setup without dedicated hardware has its own set of problems.
The iMac has two Thunderbolt ports so naturally one is taken up by the Thunderbolt Display which then has two Thunderbolt hard drives daisy-chained off of it. The iMac's second port then has a Mini DisplayPort to HDMI adapter which runs to an HDMI splitter which goes to the two broadcast monitors. It's not pretty, but it does mean he gets three Desktop screens to work with when the broadcast monitor is not in use by Premiere Pro's Mercury Transmit.
Recently this editor called me in to his bay because he realized his client monitors (which, remember, are running off the same HDMI signal) were being squeezed down and letterboxing was occurring. I tried the usual round of power cycling and reconnecting monitors, restarting the machine, and turning Mercury Transmit on and off. Nothing helped. I finally checked the settings on one monitor and noticed it was claiming the signal it was receiving was 1920x1200 which was funny because a) it should have been 1920x1080 and b) that was a 1080p monitor, so it couldn't even show all the 1920x1200 it was claiming to receive. The next stop was to System Preferences > Displays to find that yes, OS X (version 10.9.5) was sending a display signal at 1920x1200.
Clearly some part of the HDMI handshake was getting confused. However, under the list of available resolutions there was no 1920x1080. There is, however, a trick. Hold the
option key when clicking Scaled in System Preferences > Displays and OS X will show you all possible resolutions as a sort of override to what it thinks.
Finally, on that list, I found 1080p as an option. The editor could get back to work.