Since sysutils/munin-master/pkg-install uses crontab(1), the munin crontab file will always be installed at /var/cron/tabs/munin. This is an issue when installing munin-master by specifying a different root directory (e.g. for a jail via "pkg -r <jailroot> install munin-master"). The cron(8) process reading /var/cron/tabs/munin will try to execute /usr/local/bin/munin-cron, which doesn't exist, and under the different root hierarchy there won't be any crontab file at all.
I'm not really sure how to best fix this, as crontab(1) has no notion of specifying a different root (and copying the tab file around after crontab(1) created it is messy... I guess nothing should be installed outside of the root anyways, not even temporarily?)