When installing 9.3-RELEASE (also true for 9.2-RELEASE) the installer only allows 15 characters for naming mount points when creating filesystems. This is unnecessarily short. For example, something as simple as /usr/local/tinderbox does not fit into this limit. 15 characters seems arbitrarily short. I would suggest at least doubling that, if not extending it all the way to whatever the actual limit is on mount names in fstab.
It is true also for 14 CURRENT (currently MNAMELEN is 1024). I can take this PR to update the installer.