From DGG - While fixing this bug, please verify that this type of bug is not present when OTHER fields/sections are excluded from edit form via Site Preferences at civicrm/admin/setting/preferences/display?reset=1 (e.g. Demographics, Comm Prefs, Notes).
EXAMPLE: Enter values in Comm Prefs fields for a contact, then disable Comm Prefs section, edit/update the contact and verify that Comm Prefs field values are intact.
Also, test this scenario when excluding fields for Address Editing (civicrm/admin/setting/preferences/address)
So if I edit contact 1500 who is in groups 1, 2, 3 (not smart groups)
And say add or change a phone number.
When you come back to the groups list they are all removed.
This was in 1.8 and now 1.9
Here is what I did to fix it....
I went into the Global settings and Site Preferences and turned back on Show Groups/Tags while editing the Contact.
Which is not what I want people to see here for a couple of reasons, BUT without it we saw what happened.
I may report this as a bug?