Just wasted about twenty minutes on a Sudoku puzzle (from a cheap book I usually keep in the loo). My usual time for the puzzles in this book - which are all fairly easy, despite being marked "beginner, intermediate" and "expert" - is about four minutes. This one just wouldn't come out... until I realised that there were actually two possible solutions, neither of which was erroneous. Basically there were two pairs of 2 and 4, on the left side and in the middle - you could have 2 on top and 4 below to the left, with 4 on top and 2 below in the middle, or vice versa, without affecting anything else.
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!