I have an external monitor (specifically, an HDTV) hooked up to my 2.53GHz 13" macbook pro. This display works fine and I use it with my mac in clamshell mode (eg. with an external keyboard/mouse and the laptop closed and the built-in mac screen turned off)

My Mac has multiple users on it. For User A I can use the mac with the external monitor in both clamshell and dual-monitor setups.

For User B, I can use the monitor in a dual-monitor setup, but whenever I switch to clamshell mode the Mac switches to an incorrect output resolution or frequency setting that my HDTV doesn't recognize, resulting in a blank screen and a message about Unsupported Resolution.

Chances are I did this to myself by misconfiguring my display settings at some point in the past, but I have no idea how to undo it. I (obviously) can't seen the display to change the settings when it's borked. I can see the display settings if I switch to Dual-monitor mode, but those settings only affect the dual monitor setup; no matter how I change the settings in dual-monitor mode, the clamshell mode setup remains borked.

How can I dig myself out of this hole?