The text rendering system will cache bitmaps generated from the outline fonts.
It is not uncommon that this cache gets corrupted. Sometimes restarting the app is enough, sometimes the machine, sometimes the cache manually needs to be flushed.
How is it that the cache comes to be corrupted? This happens to me at least a few times per year, and it seems to be a fairly wide spread problem. None of the other data on my computer gets corrupted 3 or 4 times a year, what is the deal?