I am sorry if this has already been covered, but I could not see it there.
In the very rare instance of a single word having more than one style tag applied, cufon treats this as two words, and will split them during word wrap. For example, if you were to style the word "hello" with the first three letters as <strong> and the last two letters as <em>, like this:
Cufon will treat this as two words, and will split in the middle during word wrap. I have an example here:
If you keep reducing the size of the window until the word wrap happens at the word "single", it will split the word in two (tested in Chrome, FF and Safari).
Obviously this would be a very rare occurrence, but it is incorrect nonetheless. As there is no space in between, they should be interpreted as a single, unbreakable word.