Although Macs tend to be quite stable, sometimes things go wrong.

There may be instabilities caused by new hardware (particularly with a related utility) or memory (RAM) upgrades, if the module is of dubious provenance, have been known to cause problems.

Other causes may be software, such as third-party applications or, if the operating environment is not perfect, when updating the system. The upgrade to Leopard was an example: on two machines, I had no problems. A third, which was not used every day by me, failed. I resorted to another installation method, after which all was well.

In the Apple Discussions forums, after every update there are messages that read, "Apple, this is the worst upgrade ever", when the user's environment may have had some effect. Users rarely confess to that.

Some users add to problems by deleting or moving files they do not recognise. If you do not know, don't touch.

The worst manifestation of a problem (hardware or software) is the kernel panic: the OS is confused and gives up. Fortunately, this is rare. Some users have many, one after the other, without applying any maintenance or repair, expecting the operating system to be self-healing.

If a process stops while we are working, we may see a rainbow coloured cursor. In most cases this will stop and the process will resume. If it continues for more than a few minutes, we can click on the Apple icon at top left of the screen and access "Force Quit." This will not normally affect the operating system, but it would be a good idea to investigate further. Simply logging out of the account, or restarting the computer can clear some problems.

Sometimes, a stuck process could cause the system itself to seize, leaving us with a movable cursor but little else. Again, time may be the healer, but we may need to click on the Finder icon in the Dock to enable access to Force Quit.

If this is not succesful and considerable time has passed we may have to resort to a shutdown, although this is rare. If the menu is unavailable try the Ctrl, Option and Command keys with the Eject key to shut down. At times, not even this will work and we may have to consider the Power button.

I try to avoid this and (like a kernel panic) it is no good just continuing, as there are incomplete processes: a restart may restart the problem. At least use the Safe Boot: press the Shift key. That does not activate the startup items and, if followed by a normal restart, allows the normal cleanup processes to be completed. I always consider maintenance. This is essential after a kernel panic, athough some blithely continue. I wonder if they check the oil in their cars.

At times, this is not enough and it will suggest a stronger approach. Two applications are favoured by long-time users of OS X: TechToolPro and Disk Warrior. By the time you need these, it is too late. My preference is for Disk Warrior. Not only is it an essential standby, but I run it regularly as a form of preventative maintenance.

Using Disk Utility, from the installation disk or from the Utilities folder, we can also Repair Permissions. That may also clear some problems. I run this once a week, and also before and after installing or updating software.