Then don’t update when you are in an exam. 
Generally: the smaller the update process the smaller the chance for issues, the smaller the number of packages which could be the cause, and the less work it is to work out what broke.
To prevent “accidental” updates you can either limit your use of -y, use the ALA, or use e.g. my stepped repo (Current, Daily, Weekly, Deferred mirror setup - #5 by jonathon).
And for “ultimate” safety, install onto a BTRFS root and install grub-btrfs and either timeshift or snapper, then you can easily boot to a previous snapshot of the OS.