An unsafe value is filtered out of two read paths, but a third reader of the same rows still returns it as before. The client gets the value it was meant never to see, through the door nobody listed.
List every reader of a field before you guard one. A grep for the column finds the door you forgot.
Guard at the source when you can. One check where data enters covers every exit.
Guard at the source when you can. One check where data enters covers every exit.