This could be due to a corrupted font or a font conflict.
Launch Font Book and try to see if you have a font called
Helvetica Fractions that is enabled. If it is, disable it.
While you're in Font Book, select your fonts and click on
Validate fonts (see the attached screenshot).
And you can always download
FontNuke to clean out the font cache, in case there is some type of corruption.
Another thing : if you have many fonts installed on your Mac, it would be best that you use a more robust font manager than Font Book. I use Linotype's freebie
FontExplorer X. It checks for font conflicts, you can deactivate unnecessary fonts (which will make your Mac a bit more perkier if its resources are mobilized to deal with too many enabled fonts), you can create sets that you enable/disable at will, etc.