And if you don't want / can't figure out the shell-seg upgrade, you can also put "Set echo on" at the top of your startup sequence and comment out (; ) the "EndCLI" at the bottom. Your startup-sequence will run as normal, but you'll see everything that it's doing and the AmigaDOS window won't close at the end so that you can examine it in more detail.
If things are so bad that your system is crashing during bootup, then either of these options will allow you to see where it's all going wrong

EDIT: I should add that I used the interactive mode to diagnose some nasty bootup problems some years ago - it's extremely handy.