Mac and the OS X are very stable and reliable. However, if you want your Mac to work perfectly you have to maintain it properly. Most of the errors and glitches that happen to your mac are caused by incorrect permissions table. Mac OS X as a UNIX based system has a UNIX way of handling the files. UNIX as a multi-user system has a special way of manipulating file access. (More about file permissions you can read here). When the new software is installed on your Mac or you did some other system tweaks, it's very likely that permissions will mess itself up so you wont be able executing applications or it may happen that your mac is acting weird. The easiest way of fixing permissions is by using Mac OS X "Disk Utility" which can be found in /Applications/Utilities folder.

Fixing permissions is recommended before and after installing any software to your mac as well as installing any software updates. There are various software available on the Internet to help you keep your Mac maintained and in a "good health". Software such as Onyx or Cocktail are doing a great job by not only repairing and verifying permissions for you but also cleaning the internet history,cache,temporary files and running optimization scripts.

If you for some reason are unable to boot your system,before you do anything like re-installing the OS, try pressing command + S on the system boot-up and it will get you into console mode. Type in /sbin/fsck -fy and hit enter. Do that couple of times until it doesn't say "System Modified". Before you use "fsck" command, I advise you to open terminal, type in man fsck and read the manual page carefully. Not all software errors are as easy to fix by using methods above. Some errors and system misbehaving can indicate serious hardware failure so if the problems persist, the best is to take your mac to an authorized Apple service for a check-up.