I'm the guy who typeset this, and I'm ashamed to have missed that.
I know exactly how that happened though. "M" is a hotkey for the "move" tool in GIMP. I meant to move the bubble but accidentally put a letter in the bubble.
Thanks for catching that. I wasn't really happy with some bits of page 8, so now I have an excuse to fix that as well.
Let me know if you have any more suggestions.