Holy crap, three posts in one month?!?!? Yeah, what can I say. This is something that sometimes happens to us letterers (well, it happens to me sometimes), but it's usualy caught at some point before the book is printed or otherwise published. To this letterer's (and editors, etc) credit, the lost balloon tail is white on stormtrooper armor, so it's not that easy to spot.
(From Star Wars: Purge - The Tyrant's First 2 of 2, letterer Michael Heisler.)