This page describes troubleshooting common crashes which occur outside MythTV, usually X or at kernel level. They are not caused by MythTV but usually by bad drivers or bugs in third party hardware.
Frontend Crashes X Server on Startup
Problem: Whenever starting mythfrontend on a separate front/backend system setup, with X up and running, the screen would go black momentarialy, then return the user to the login screen. The X session had completely restarted.
Solution: This appears to happen some times when the GLX portion of the Nvidia drivers have been overwritten by updated Xorg pacakages. Re-installing the NVIDIA drivers either via NVidia.com's packaged installer or the distribution's installer has been reported to resolve this problem.
Ubuntu specific solution: Some posts over on the Ubuntuforums.org got a reply that someone with a similar problem reinstalled their video drivers and it went away. In my case, I used envy  to reinstall my nvidia driver for my XFX5200 card, and the crash went away. I believe envy checks lots of dependencies in order to get the most correct and up to date drivers installed--worked for me!
Another method of trying to diagnose a mythfrontend failure would be to run, in a terminal window:
mythfrontend -v all 2>&1 > /tmp/mythfrontend.log
The above command will grab all debugging output from mythfrontend and place it in /tmp/mythfrontend.log for the entire session that mythfrontend runs. Be aware that you do not want to run your frontend this way for very long since the resulting log file will become very large.
If that does not yield the desired results, a desperate diagnosing attempt would be to install "strace" and run:
strace -f mythfrontend 2>&1 > /tmp/mythfrontend.log
This will output a LOT of detail from the system level of what mythfrontend is doing (opening file handles, host name look up, opening tcp ports, loading libraries, etc). You really should only do this method if you are truly desperate.