What do you mean by "change a permission?" I have REPAIRED disk permissions, but never changed one to my knowledge. If I did change one, it did not adversely affect my MBP. If it's a setting you have to actively search out and change on your own, then I stand by my statement that you have to try harder to mess up a mac. Can one accidentally enter that command? I've never seen nor even heard of it and I've done a fair amount of work on my 4.5 year old MBP.
I'm not saying it can't happen or that macs are impervious to problems...I'm just saying they are not NEARLY as vulnerable to problems as PCs are. Granted, I realize the sample size for PC owners is MUCH higher than mac owners, but every time someone calls me with a computer problem it's always a windows computer. When mac owners call me it's more of a "how do you do this" type question. Not "How do you fix this?"
When you factor all the adware, spyware, malware, viruses, trojans, worms, etc. that PCs can fall victim to I firmly believe PCs get screwed up much more easily than Macs. And like I said earlier, I know macs can get a virus, but the threat is so slim right now it's not worth worrying about.