The following plain tex example
$\left( A \middle| \right)$ %OK
$\Uleft( A \Umiddle class 2 | B \Uright)$ %OK
$\Uleft( A \Umiddle class 2 | \Uright)$ %oops
! This can't happen (mathspacing).
l.10 $\Uleft( A \Umiddle class 2 | \Uright)$
! ==> Fatal error occurred, no output PDF file produced!
I'd actually meant to have the A|B but the error message for my typo
seemed a bit harsh!
Having an empty math list to the right of a class 2 \Umiddle might be
odd but shouldn't be an error I assume?