First time see that a couple minutes ago. One day I deleted whole OS without any prompting (rest in peace Ubuntu 10.10).
Trying to delete /jail which have base.txz I got an error “Operation not permitted”. According what man rm told me, I follow next stage and looked through man chflags, found -sunlink (system undelete). But I go further and just execute:
chflags -R 0 /jail; rm -rf /jail
Just to remember.