OS X Error Messages and Symptoms
Just like an other Operating System (OS), Mac OS X can encounter errors that cause it to lock up, or on the very rare occasion, crash.
The most common cause of these errors are related to issues with third-party applications that have entered an infinite loop and/or consumed all resources on the computer preventing the OS from responding. This type of error has occurred quite frequently with Flash plugins in Firefox and Safari browsers, for instance. The solution to app-related issues is to first try upgrading the offending application or plugin, and if that does not work, to reinstall the application. If both of these fixes fail to work, then the application is most likely too buggy to be used and should be removed.
The other potential causes of these errors are hardware issues such as a damaged hard drive, improperly seated RAM or faulty RAM. Replacing or reseating the RAM or repairing or replacing the hard drive will solve those errors if investigation indicates these to be the cause.
A final cause of these errors may be due to a needed firmware update related to the graphics card. Running software update can identify whether there is an available firmware patch for your Mac.
"Black Screen" errors
These errors are most often related to problems with RAM, the graphics card, or in rare instances a damaged hard drive. They are very rare and are almost always caused by a hardware issue. In all instances where these errors occur, the RAM should be reseated and a hardware scan run on the computer.
The Spinning "Beach Ball"
The spinning wait cursor or "beach ball" is the Mac's version of the hourglass cursor. It indicates that the system is busy and cannot respond right now. This beach ball cursor may appear when you are running very intensive tasks such as rendering videos or if you have many applications running at once and system resources are low. It can also appear momentarily when opening some apps or saving some large files, etc.
If the "beach ball" appears frequently on your Mac, this may indicate that you need to install additional RAM or require a new drive, particularly if it is several years old or you are regularly running intensive applications such as Adobe CS5, iMovie, virtual machines, etc. If the beach ball won't go away and there is no obvious reason or it appears to have locked the computer, then this is likely an indication that an application has entered an infinite loop and needs to be force closed (see "Force Quitting Applications" below)
Force Quitting Applications
If an application refuses to quit, or you are experiencing the endless "beach ball" for no apparent reason, you may need to be a bit forceful. To "force quit" an application in OS X use the "Command-Option-Esc" key combination to bring up the window above. Once this window appears, select the troublesome application (it will often be highlighted in red and say "not responding") and click the "Force Quit" button.