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 \bye produces ! This can't happen (mathspacing). l.10 $\Uleft( A \Umiddle class 2 | \Uright)$ %oops ! ==> 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? David