Sometimes the OPAC does go down and people want to check out books, as illustrated in yesterday's Unshelved.
One temporary solution is to open up a Word document and scan the barcodes on the patron's card and books into Word. A savvy staff member may be able to write a macro to import these back into the database. If not, cutting and pasting from Word isn't so bad.
But if the power goes out and you have to handwrite everything? Another reason every library should have a laptop for staff to use. Plus you can carry it around the library when all the computers are in use and you need to train someone on database. I almost used my personal laptop for this reason, but I think the Mac would have confused some patrons.