The ability to look back on your work and acknowledge where improvement is needed is a great asset to have in your toolbox. We all know we're not perfect, but kudos for having the humility to call yourself onto the carpet in a way.
This specific scenario is interesting - when you say "something got messed up in QLAB" - I assume there was not a live backup running? Others here have said it's always good to have a CD backup - while I agree that having a backup is necessary, the ideal way to do this is to be running a live backup QLAB machine that is a separate piece of hardware from main show machine. Both machines should be dedicated QLAB machines (not peoples' personal laptops, and not machines that do other duties). If (when) a crash occurs, a change is as simple as bringing the levels on the mixer down for the primary and up for the live backup, and pressing a button on the KVM from primary to backup.
A redundant QLAB setup with two MAC Minis, Monitor, KVM, Keyboard, Mouse, and Software License can be had for under $3,000 (if you've got the right audio system to accept it - a touch more, say $5,000 if you need added hardware). I realize many theatres operate on shoe string budgets, but if this is an insurmountable sum, probably best to find a solution other than relying on QLAB.
So often we are reactionary and say "what should I do if X situation occurs?". Look to be proactive during your tech run and say "how do we avoid X, Y, Z situations?". If your sound designer or audio engineer's answer to a possible failure is "let's hope it doesn't happen!" - that's something to work out way before showtime.