From https://bugs.launchpad.net/ubuntu/+source/gnome-mount/+bug/178154:

Specifying mount options in Nautilus' volume properties does not have the expected behavior:

Steps to reproduce:
1. Plug a removable drive with a VFAT partition.
It gets mounted with the `rw,nosuid,nodev,shortname=mixed,uid=1000,utf8,umask=077,usefree` options.

1. Go to ''Places/Computer'', right click on said removable drive;
2. Expand the ''Settings'' part of the ''Volume'' tab;
3. Add `quiet` as ''Mount options'';
4. Demount then remount the device using ''e.g.'', Gnome's popup menus;
{X} The volume is now mounted with the `rw,nosuid,nodev,quiet` instead of the expected `rw,nosuid,nodev,shortname=mixed,uid=1000,utf8,umask=077,usefree,quiet` set.

1. Now specify `rw,nosuid,nodev,shortname=mixed,uid=1000,utf8,umask=077,usefree,quiet` as mount options in the ''Volume'' property tab;
2. Demount then remount the device;
{X} An error message box, appears saying ''Invalid mount option when attempting to mount the volume 'VOLNAME'.''.

Additionally, the same behavior can be triggered using more than one `mount-options` with `gnome-mount`:
`/usr/bin/gnome-mount --hal-udi /org/freedesktop/Hal/devices/volume_uuid_WHAT_EVER --mount-options=rw,nosuid,nodev,shortname=mixed,uid=1000,utf8,umask=077,usefree,quiet` whereas the MAN page for `gnome-mount` states:
           -o, --mount-options
                  Specify mount options. Separate by comma.
NB: it does work with only the `quiet` option.

After fixing this bug, it would be interesting, IMO, to extend the `--mount-option` feature to allow for addition/removal of options from the defaults.
For example, for a device being mounted with default `rw,nosuid,nodev,shortname=mixed,uid=1000,utf8,umask=077,usefree`, specifying `-utf8,+quiet` in the ''Mount options'' entry would result in it being mounted with options `rw,nosuid,nodev,shortname=mixed,uid=1000,umask=077,usefree,quiet` (that is the default set minus `utf8` plus `quiet`). Not putting any plus or minus sign would result in the mount options being completely replaced.

Additionnaly, more in-line documentation (popup label and help file mentioning, at least, the need for comma separation) should be accessible to avoid "normal" user to resort to MAN pages to understand how to fill the ''Mount options'' entry.