A couple of months ago I wrote that, much as I love using it, Mac OS X:
has so many dependencies, so many thousands of files, and so many intricacies, that manually fixing a choked-up system is close to impossible. [It] may be capable of running for weeks or months or years without a reboot, if something does go wrong, it can go catastrophically wrong, and it's hard to know why.
John Gruber, who's always worth reading, agrees about OS X's sometime brittleness:
When things go wrong on Mac OS X, they often happen at a deeper level. File permission and ownership problems, for example, are not something a typical Mac user can deal with. It's not enough that Mac OS X doesn't require any Unix nerdery whatsoever for day-to-day use—it should never require Unix nerdery to recover from software updates, either.