I accidentally ran mergemaster -pv as a normal user instead of root and this is the output:

([J:0]jnbek@rtr10[~]% mergemaster -pv                                                                                                                         [08/25/15][19:39:56])

 *** You have /usr/bin/less defined as your pager so we will use that

*** The directory specified for the temporary root environment,
    /var/tmp/temproot, exists.  This can be a security risk if untrusted
    users have access to the system.

  Use 'd' to delete the old /var/tmp/temproot and continue
  Use 't' to select a new temporary root directory
  Use 'e' to exit mergemaster

  Default is to use /var/tmp/temproot as is

How should I deal with this? [Use the existing /var/tmp/temproot] d

   *** Deleting the old /var/tmp/temproot

*** Creating the temporary root environment in /var/tmp/temproot
 *** /var/tmp/temproot ready for use
 *** Creating and populating directory structure in /var/tmp/temproot

 *** Press the [Enter] or [Return] key to continue
install: /var/tmp/temproot/etc/group: chown/chgrp: Operation not permitted

  *** FATAL ERROR: Cannot copy files to the temproot environment

([J:0]jnbek@rtr10[~]% ls -R /var/tmp/temproot                                                                                                                 [08/25/15][19:41:40])
etc/

/var/tmp/temproot/etc:
master.passwd

This seems to be a bad thing. It's a stock master.passwd file but I would think that mergemaster should fail before ever messing with this file in the first place unless the user is root. If a unprivileged user gets compromised on a system, this could be used for evil, although I'm not sure exactly how.