The Problem
If you have the Helvetica Fractions font installed, text in some applications may appear as overlapping numbers and symbols. This font substitution problem can effect many Mac OS X applications including Address Book, iChat, Safari, Mail and Microsoft Entourage.
The Fix:
Remove Helvetica Fractions from your font collection. Or if you can't entirely remove it, deactivate it manually when this problem occurs.
In practice, this may take a little hunting to accomplish. Often Helvetica Fractions is included as part of a larger Helvetica font suitcase and can't easily be removed without rebuilding those suitcases. The good news is that few designers actually use Helvetica Fractions in their daily work and this font generally won't be missed.
Tip:
If you are managing your font library with FontAgent Pro or Suitcase Fusion (using the Font Vault feature) you should be able to remove the individual Helvetica Fraction fonts without disrupting other Helvetica's in your font library.
More details: Incorrect Font Substitutions
Details:
What is a font substitution problem? It’s very simple: when a document or a webpage calls for a specific font and due to unavailability or inadequate font calls to the OS another font is chosen instead, an incorrect font substitution results. For the most part this isn’t the worst problem to have with a browser or your e-mail application. If a webpage calls for Arial and somehow Arial Narrow (another normal roman font) is used, you would still be able to read the text. If you have Helvetica-Fraction or Helvetica-FractionBold present in either your font manager or in one of your various OS Fonts folders, then this font could, and often does, get used instead of the normal Helvetica font specified.