I had never noticed this on my Kobo Touch until you'd mentioned it. I don't really like to read long passages of text with full justification, having the same amount of spacing between each word is easier on my eyes. That said, I do subscribe to a newspaper on my Kobo, and they force me to read that fully justified.
With fully justified text, I can see a tiny, tiny bit of overhang for words ending in the lowercase letters "f" and "g". The top of the f goes slightly over too far, and the bottom of the g is ever-so-slightly a tiny bit too far.
But when I examine the body of the text, I see that the same two letters ALSO push exactly the same distance into the next letter.
From what I can tell it is a side-effect of they way the Kobo interprets the font's kerning. Here's a quickie link to wikipedia's Kerning article for a bit more info:
http://en.wikipedia.org/wiki/Kerning
Depending on which font you use, its kerning may be more or less pronounced. In the font I'm currently using, if a word ended in a capital Q it would probably overhang the right margin by the greatest amount.
And I agree that this is actually a bug in the rendering of the text: any word which, when fully rendered, overhangs the right margin should shrink the interword spacing on that line, or else if it cannot be shrunk further then that word ought to be wrapped to the next line.