Why are the file permissions in OSX this often wrong and have to be repaired?

Question

The first thing (or second, after a clean re-boot) to do when you have problems with your Mac OSX is "repair permissions". This often helps for minor problems.

But why are these permissions this often wrong? What is the reason for them being incorrect?

Answer 1

Apple's knowledge base gives a number of reasons permissions could be screwed up. See the "Issues Related to Permissions" section.