I don't know if this is a clue, but my exit produced garbage, and a segmentation fault that took the enclosing bash process down. Here's what I saw. I was running inside of bash, inside of emacs, which was running as root.
corn:/usr/local/src/tools/apt-tmp# sudo -u www-data ../apt-cacher-1.6.8+nmu2/apt-cacher2 daemon_port=3333 -d& [1] 12590 corn:/usr/local/src/tools/apt-tmp# debug [12590]: Using IPv6 debug [12590]: Listening on :::3333 debug [12590]: fork listener debug [12590]: Clean up before exiting. C-c C-c [1]+ Done sudo -u www-data ../apt-cacher-1.6.8+nmu2/apt-cacher2 daemon_port=3333 -d corn:/usr/local/src/tools/apt-tmp# bash: èÂ: command not found corn:/usr/local/src/tools/apt-tmp# bash: Â: command not found corn:/usr/local/src/tools/apt-tmp# bash:ÃÃò¿ÂñîuhèÂ: command not found corn:/usr/local/src/tools/apt-tmp# bash: syntax error near unexpected token `1' corn:/usr/local/src/tools/apt-tmp# bash: 8B: command not found corn:/usr/local/src/tools/apt-tmp# bash: ¨á: command not found corn:/usr/local/src/tools/apt-tmp# bash:Â: command not found corn:/usr/local/src/tools/apt-tmp# bash: Ãg: command not found corn:/usr/local/src/tools/apt-tmp# bash:  ®: command not found corn:/usr/local/src/tools/apt-tmp# [1] 14633 bash: : command not found bash: #ð¨PI: command not found [1]+ Exit 127 corn:/usr/local/src/tools/apt-tmp# bash:  : command not found corn:/usr/local/src/tools/apt-tmp# bash: Â: command not found corn:/usr/local/src/tools/apt-tmp# bash: Â: command not found corn:/usr/local/src/tools/apt-tmp# corn:/usr/local/src/tools/apt-tmp# bash: x: command not found corn:/usr/local/src/tools/apt-tmp# bash: ¤: command not found corn:/usr/local/src/tools/apt-tmp# bash: Ã: command not found corn:/usr/local/src/tools/apt-tmp# bash: : command not foundõHÃò¿Âà µñî?±ÃjôÃò¿�...@: command not found corn:/usr/local/src/tools/apt-tmp# bash: þÿÿÿ¨}: command not found corn:/usr/local/src/tools/apt-tmp# bash: Âd: command not found corn:/usr/local/src/tools/apt-tmp# bash: @$ë·0¨}: command not found corn:/usr/local/src/tools/apt-tmp# bash: ȍ: command not found corn:/usr/local/src/tools/apt-tmp# bash: XI: command not found corn:/usr/local/src/tools/apt-tmp# > > > > Process shell segmentation fault -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org