Occasionally (every few weeks), a user comes along to me complaining that the fonts in one of their applications has become "garbled" or "corrupted." Safari seems to be the subject of most complaints, but I have also seen this occurring in Firefox, and I'm pretty sure I've seen it before in other applications as well. The symptom is simply that an incorrect font is used for the display of text. For example, Cursive being used instead of Times. In sections of text where the style changes to bold and/or italics, the font may or may not switch to the correct one. If a symbol font happens to be the incorrectly-displayed one, it looks sort of like hieroglyphics -- hence the description of "garbled" or "corrupted" text sometimes seen on various forums.
Note: I don't think "corrupted font" is an accurate characterization of the problem. The fonts themselves don't appear to be corrupted; rather, the system is using the wrong font to display some text. The following solution worked for me, without requiring a reboot or logout. It can be done as the user experiencing the problem (ie: no admin access is required).
Mac OS X Hints