This test K2-OrderbyExprWithout-19 reads:

let $i := (1, 3, 2)
order by $i
return $i

and is supposedly invalid because the sort key is not a singleton. However, I think an optimizer is allowed to avoid evaluating the sort key when there is only one tuple in the tuple stream. I'm not sure I can find an explicit statement to that effect, but it's very much in the spirit of the rule that you never need to do extra work at runtime whose only purpose is to detect errors.