When you are changing the permissions for a role in the Administration section sometimes you get an error dialog.
Go to Administration > Roles and edit the permissions of a role. Click change for any one of the sections click allow for some of the activities and sometimes you get a dialog saying.
The page at www.joineryproducts.com.au says:
You are trying to access item that no longer exists. The most probable reason is that it has been deleted by another user.
Hitting ok and then change again will in most cases work. I could not consistently cause this erro but have seen it about 5 times.