Packages misbehave, system crashes in the middle of the experiment, and you stress out because you have to cancel on your participants till you find a fix.
This is especially important if you are using a department system. You can leave a note on the system's desktop reminding others not to install any system updates or better yet, install "Sledgehammer" to halt them (from a very informative RA who taught me how to do it today who has been using it for some years without a hitch).