This is something I have to do with on a daily basis. The list sounds silly, but is spot on:
usually we are affected by all of them
This lad is an artist. I love his Norman Rockwell themed social critique like

Earlier this year I was faced with the challenge to upgrade a Mac Book Pro to Leopard, the machine in question would not read the Leopard DVD, however. What to do, go back to the Apple store and lament the oddity?
Well, fortunately I had a second Mac Book Pro from work which I upgraded prior and which could read the DVD just fine, which also told me that the DVD media itself wasn't to blame.
I remembered that you can hook up two Apple computer, using the disk of the other computer by booting it in Firewire Target Mode. Doing that I was able to use the DVD drive of the computer that was able to read the Leopard DVD media and kick off an installation on the disk in the other computer.