Dear devs,

   ! error:  (nodes): attempt to double-free kern node 239, ignored
   !  ==> Fatal error occurred, no output PDF file produced!

MWE:

   \catcode`@=11

   \Umathordordspacing\displaystyle=\thinmuskip
   \Umathordordspacing\textstyle=\thinmuskip

   \def\operator@font{\fam0}

   \def\qopname#1#2#3{%
     \mathop{\mathpalette\@qopname{#1\kern\z@\operator@font#3}}\csname n#2limits@\endcsname
   }
   \def\@qopname#1#2{%
     \hbox{$
       \Umathordordspacing\displaystyle=0mu
       \Umathordordspacing\textstyle=0mu
       \m@th#1#2
       $}}

   \def\max{\qopname\relax m{max}}
   \def\min{\qopname\relax m{max}}

   \catcode`@=12

   $$\max a b / \min a b$$

   \bye