That makes it all the more vexing when minor things go wrong that seem pretty unexplainable. One little problem is Apple Mail notification sounds. They seem to work about half the time. I have 3 email accounts, and the one that is the most unreliable is the MobileMe account. Even though mail sounds are turned on, I usually don't get any sound when mails comes in. I use the 'glass' sound, but it fails on any selection. My POP mail accounts seem to trigger the alert properly.
This same problem existed in Tiger, and has survived several Leopard updates. This minor but irritating issue exists on both my computers, and so far has eluded all my detective work to eliminate it. This bug is impervious to permission resetting, changing default mail sounds, playing with the sound control panel, (yes, 'play user interface sounds' is checked) and making sure that Growl and other 3rd party hacks are nowhere on my computers. The guys at the Genius Bar scratch their heads and smile.
The Apple support boards are full of similar complaints, and when you eliminate the people that had their sound turned down, or the other obvious fixes, there is a pretty large residue of people who have this problem. Apple must be aware of it.
Don't get me started on the error I get 2-3 times a week when Time Machine tells me a backup didn't work, and then the next hour everything is fine. This is another error that has survived several Leopard updates. Like a good citizen, I always do the larger combo updates to have the best chance of replacing any bad system components.
Every OS has a long list of issues. Actually, I find the current build of OS X the most reliable system software I have ever used. That said, I'd love to see some of these little bugs cleaned up. I'm sure you have your own lists and will share them.
Maybe Apple will read this and send me a fix for my notification sounds issue. The problem is, I probably won't notice the mail when it comes in. Grrrrr.