What version are you running?
2.5.9

What's the URL of the page containing the problem?
http://1.2.3.4/rb/admin/db/reviews/group/3/

What steps will reproduce the problem?

Manage a review group
click a "x" next to a user to remove that user
Click save
error message: "Select a valid choice. 33 is not one of the available choices."


What is the expected output? What do you see instead?
Would expect the group to save without the user(s) removed. getting an error message instead.

What operating system are you using? What browser?
Ubuntu 14.04, Chrome latest

Please provide any additional information below.
In fact even clicking save without changing any information seems to trigger the error?!?!?!