French wikipedia say:Un processus en sommeil ininterruptible ne peut s'arrter (ni librer ses ressources) mme lorsque SIGKILL apparat.   Il s'agit d'un des rares cas dans lequel un systme UNIX doit tre redmarr afin de rgler un problme logiciel temporaire.In english (c/p from old revision of english wikipedia):An uninterruptibly sleeping process may not terminate (and free its  resources) even when sent SIGKILL.This is one of the few cases in which a UNIX system may have to be rebooted to solve a temporary software problem.It imply that there is other case than uninterruptible sleep than can need a reboot to solve temporary problem, I thought it was the only one.What are the other cases?
Is uninterruptible sleep the only case?
kill;sleep
This answer explains the un-interuptable process and leads to a couple of external articles for more insight. StackOverflow Answer