In the real world there are always going to be failures, triggered by things like software bugs, hardware failures and misconfiguration. Ubuntu should where possible handle common failures and provide predictable feedback to the user that the system is broken. These failures should also be detected by the system and the information available via Apport so we can know how frequent these cases are.

This is a list of failures we might expect to occur (in approximate order of occurrence from boot), and what we should do to help the user:

Missing / Corrupt Master Boot Record (MBR)

User uses live CD to recover (this instruction given from a FAQ or similar on the web / documentation provided with the CD). Live CD should detect if broken and give user options / information that can fix it. (Probable causes are hard drive failure, other OS installed over MBR).

Root filesystem not valid

User is shown SMART information about drive. (Probable cause is a hard disk error).

Unable to run kernel

User is returned to the bootloader and it goes into a recovery mode. (Probable cause is an upgrade failure).

Corrupt initramfs

User is returned to grub and previous kernel is suggested. (Probable cause is an upgrade failure or hard disk error).

Filesystem check detects corruption

User is prompted to manually fix filesystem. (Probable cause is hard disk error or power removed while writing).

Packages in partial upgrade state

User is prompted to finish installing and upgrade packages in a text failsafe mode. (Probable cause is power loss during previous upgrade).

System services not started in reasonable time

User is sent to a failsafe graphical mode, i.e. does not wait for services to start. (Probable cause is faulty software).

X server does not start

User is sent to a failsafe graphical mode. (Probable cause is unsupported video hardware or bad upgrade).

Graphical failsafe mode does not start

User is sent to a failsafe text mode. (Probable cause is broken video hardware or bad upgrade).

Greeter does not start

User is sent to a failsafe graphical mode. (Probable cause is a missing or faulty greeter).

Session does not start

User is returned to greeter, information about why cannot run session displayed. (Probable cause is bad upgrade to session packages).

Unity 3D does not start

User is automatically switched to Unity 2D. Message displayed saying why can't use 3D mode. (Probable cause is unsupported video hardware).

Compiz/Unity crashes during session

Compiz/Unity restarted automatically and all windows placed in the same locations. This event should not be visible to the user except for a slight delay when it occurs. (Probable cause is bug in Compiz/Unity).

Cannot connect to network

User is notified via Notify OSD message. (Probable cause is network not available).

Application does not start

Unity gives error message why application can't start (Probable cause is a faulty application).

Unable to load web page

User is notified via error page in web browser. (Probable cause is ISP not connected).

Upgrade does not complete

User is told upgrade has failed (which packages failed to install) and system attempts to recover what it can. (Probable cause is a bug in a package).