It seems like everything having to do with computers only gets more complicated and more fraught with technicalities over the years. (See "The Tangled Road to a Universal Font Format.") Not so with fonts. They have become far easier to use and far more reliable. Most font errors come from sloppy housekeeping habits--that is to say, they're your fault--but some others are out of your control.
Crummy-looking, lumpy type. This happens when your printer can't find the necessary printer font and prints a smoothed version of the type as it appears onscreen. It means you have the screen fonts installed but not the printer fonts.
Courier substitution. This is a variation on the theme above. It can happen when the printer fonts you need are not installed or when you get a document from someone else and you have neither the necessary screen fonts nor the printer fonts installed.
Wrong typeface. There is a remote possibility that two fonts will have the same internal ID number (applications usually handle fonts by number rather than name). In this case, the wrong font may appear. Most software is smart enough to prevent this by issuing its own temporary ID numbers to avoid duplications. Font managers do the same thing. If this happens to you, the only solutions are to keep only one of the problem fonts on your Mac at a time or to buy a font management program (such as one mentioned earlier) capable of renumbering fonts to avoid ID conflicts.
Corrupt screen fonts. Screen font files carry vital data about how the System and your programs should handle type. Your Mac reads these files over and over every day. Sometimes in this process they become damaged, or corrupted in the jargon of high-tech. Corrupt fonts can cause an amazingly diverse range of problems, few of which seem related to type: System freezes, crashes, and corrupted documents that your applications will refuse to open or read and that may crash your System when you do try to open them. Some font management programs and font utilities can weed out corrupt screen fonts. But if you don't have such a program, you have to find the culprit by trial and error. If you remember which fonts you were using when the problem first appeared, try removing them from the Fonts folder and replacing them with fresh versions from the original disks. If that doesn't work, you may have to remove all your fonts and replace them from scratch.