Steps to reproduce:
Navigate to the groups of a user. Users -> [user] -> User Groups
Click the "+" button next to a group to which they are not a member.
The user is added to the group, and the page now shows the groups the user is a member of.
The user is added to the group, but the page does not reflect this. Refreshing the browser does not help
Note: The user /is/ part of the group, and can be seen when viewing the users in that group i.e. Groups -> [group] page. It is only the groups for that user that is not working - I suspect a cache somewhere needs updating.
This seems to have gone away, possibly because of a number of other fixes in this area.