how to scroll up a boot message missed

Since Karmic there is no Bootlog and is really no big issue. 

It is just helpful for troubleshooting actual boot problems.

When you see a failed message scoll by just place a link to /sbin/sulogin in /etc/rcS.d or /etc/rc2.d shortly after the problematic script, ie.
sudo ln -s /sbin/sulogin /etc/rcS.d/S81prob

Probably it's wise to take away the 'quiet' and 'splash' kernel parameter the next boot.

The link will give you a root shell, then you can use the shift + PgUp to scoll back.

voilà