To say that I'm annoyed is putting it mildly - these things are supposed to have ONE unique solution. But someone messed up the error checking and this is the result. The real bummer is that I can't now trust the rest of the puzzles to be OK, so I might as well bin the book.
Later: Found another one, this time with four pairs (on horizontal lines 4 and 5) which give a valid solution either way if they're all flipped vertically. I'm now finding looking for the errors more interesting than doing the puzzles!