When upgrading your operating system, expect the worst. Expect that your system won't boot. Expect your favorite applications won't run. Expect that your essential documents will be deleted or inaccessible. Also, your dog will get pregnant, the milk in your fridge will go sour, and you'll wake up with a big zit on your nose and run into your high-school sweetheart later that day.

Probably none of this pessimism and preparation are necessary when upgrading to Leopard. You'll probably find that the upgrade is smooth and uneventful, requiring about two minutes of work on your part, and an hour or so of hanging around while the automatic installer software does its work. That was certainly my experience upgrading to Leopard, and it's been the experience of the overwhelming majority of people who've been discussing the upgrade on Twitter and various other online communities I've been watching.

But I am completely and utterly superstitious about these matters. I am convinced, in defiance of all reason, that the reason the upgrade went smoothly for me is because I prepared for the worst first.

Also, I was wearing my lucky socks.

John Gruber at Daring Fireball has a pessimist's guide to getting ready for the Leopard upgrade. He recommends doing exactly what I did: Before upgrading, make a complete backup of the entire Mac hard disk using SuperDuper. And he recommends going further than I did: Before upgrading, test the backup to be sure it's complete and up-to-date, and unplug the backup drive from the Firewire port. "The odds of an OS installation corrupting a plugged-in FireWire volume are very small. The odds of an OS installation corrupting a FireWire volume that is not plugged in are zero," he says.

Most users will be happy using the Leopard install DVD's default upgrade procedure, Gruber says. Just follow the instructions, and sit back and relax. (I read several pages of David McCullough's excellent soldiers' history of the American Revolutionary War, 1776, while the upgrade was under way.)

If you have special circumstances -- if you've been seriously modifying the 10.4 kernel or installing developer seeds of Leopard prior to its official release -- then Gruber recommends using the Archive and Install procedure to give yourself a factory-fresh Leopard to work with.

So what can go wrong with Leopard?

Possibly the most serious problem: the Blue Screen of Death. Yes, Apple lovers often criticizing Microsoft for stealing features from the Mac, but here Apple has gone and stolen a feature from Windows! Some Mac users are experiencing the good ol' BSoD when trying to upgrade. As reported by my stalwart colleague Paul McDougall, Apple has posted a workaround. The problem seems to effect users of Application Enhancer (APE) software from Unsanity The blog switchingtomac.com has a fix for the bluescreen problem that seems simpler than Apple's recommendations. A notice on the company home page says, "please make sure you have Application Enhancer 2.0.3 or later installed before you upgrade to Mac OS X 10.5."

The Unofficial Apple Weblog provides a list of six reasons not to upgrade to Leopard: If you have only one computer and it's your production machine, if you rely on Adobe software, if you rely on Windows networking, and if you have a lot of system customizations.

Probably nothing will go wrong when you upgrade to Leopard. Probably you'll just put in the DVD, put your feet up, and start enjoying Leopard an hour later. But disaster might befall you, and it's best to be prepared.