Another good question, but a bit close to the one posted 6 days earlier - I was catching up on QOTD today after a while of not having time so I got to these two questions maybe 15 minutes apart (would have been less if I hadn't spent time thinking about the first one). If I hadn't seen the previous one, and read the references in the explanation and played a bit with output clauses to get a better feel I would probably have guessed this one wrong (picked the syntax error instead of another error).