I think the #1 reason for this is that it does not only appear very shortly after the save button is clicked, it also disappears relatively shortly after that. I click save, immediately switch tabs to test, and when I came back the error was gone.
Errors like this should, IMO, be (much) more intrusive (popup modal) or have a large red bar. And they should never automatically disappear.
Understood. In this case I think it’s better if the error never appears at all unless the client JS is changed or fails in some way. The PR will mean that you won’t see it in the normal course of operations.