Note: This is work in progress. It aims to provide quick answers to common problems from the forums and irc chats. Hopefully it can be a useful reference, an one included in the main docs. It could become CommonProblemsGuideSpec.
Brown Screen of Death
If you are left on a brown screen after logging, you maybe encountering the UbuntuDateBug.
Freeze at log in screen
Ubuntu boot to log in screen, but you are unable to type username and password.
This is likely to be a graphics driver problem. There are some specific systems this is common on.
Macs using ATI Rage 128 (CRT iMacs, some G3 iBooks and PowerBooks) http://ubuntuforums.org/showpost.php?p=501386&postcount=4