at(1) doesn't properly handle being invoked from a path containing spaces
If you happen to submit an at job from a path that contains one or more spaces, it does not properly quote/escape the path when it writes out the info to /var/spool/cron/atjobs/XXXXXX.a (after expanding the prototype in /etc/cron.d/.proto). E.g.
mkdir "/tmp/dir with spaces"
cd "/tmp/dir with spaces"
echo /bin/true | at now + 5min
If you examine the resulting job, it will contain a line cd /tmp/dir with spaces
which will of course cause 'cd: no such file or directory: /tmp/dir' in the output.
It should just be a matter of escaping the output at usr/src/cmd/cron/at.c:681