Sooner or later, every story runs into a little hiccup.
As you write, you’ll discover that certain facts don’t fit together anymore.
Maybe a character needs to be changed or removed.
Maybe you find a plot hole big enough to drive a Mack truck through.
Somewhere, in the inner workings of your story, something has gone awry.
You need to fix it, or you’ll have a big problem on your hands.