File Error
Occasionally an error in a file will prevent it from opening. Sometimes, the software will freeze or close unexpectedly. This will usually mean that you need to use a backup copy of the file. If it is a word processing document, however, you may be able to open it in a text editor and retrieve some of the text that way. The simplest way to do that, would be to drag it to your text editor's icon to start the editor and open the file within it.
The most frustrating piece of this problem is that the file will often have been opened and working fine relatively recently, but the last save seems to have damaged it in some way. That was a common problem with floppy disks, and less of a problem with other media. However, this is a good place to remind you not to open files from the server, but to copy them to the computer you are using, then copy them back to the server when done. This will prevent file errors from occuring if the server gets restarted, freezes, or has some other problem while you are working.
