select a formatter with custom settings; click edit cogwheel
Add an invalid value. (I have no example from the top of my head; I encountered this in a custom formatter that has a dropdown with an "empty" value, but that is still required.)
Press "Update" button; see error
Fix the error and press "Update" to save correctly. (Also works when pressing cancel? I don't know.)
Save the CE display. You can't, because the previous error is repeated. Only a second press of the "Save" button (after the screen was reloaded) works.
Alternatively, instead of saving anything: navigate away from the screen. The next screen will re-show the error message (once).