Oh, there are contingency plans for that. They're pretty easy. Writing down barcode numbers and then doing a mass "check out" event on a whole list when power is back covers things.

In an extended event, some backup supplies on-hand from Brodart to handle things Jersey-style with temporary card pockets is easily doable.

Redundancy is actually a good argument for maintaining a library card system along with a digital system. Don't know why the author let that one fly by.