I think this is new in v2.3.0.beta5 +43, but can’t guarantee, it’s only today that I noticed it (actually, it was a user who complained that they were locked out).
To reproduce:
Chose a user, go to the admin edit interface, modify their group membership, but don’t save (i.e, you get distracted and forget).
Chose another user, go to the admin edit interface, you will see the group memberships of the previous user, which (oops!, looks like I forgot to save something!) will be applied to this user if you save.
