Contradiction
The door in the hill is green in chapter 3 and red in chapter 12.
“the green door stood half-hidden in the bracken”
“painted a red that nobody remembered”
Nothing between the two scenes repaints the door or explains the change.
Catch it before your reader does.
Green door in chapter 3, red door in chapter 12. Your reader will notice, so the Continuity Checker makes sure you do first. It reads your whole manuscript and points out details that change, even a hundred pages apart. It never rewrites a word.
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Mara carries an iron knife through the door, though no iron can cross its threshold.
“No iron passes the hill door. Old Teg said it as if it were weather.”
“her father’s iron knife still at her belt, Mara stepped through”
The story sets this rule early and keeps it everywhere else.
Possible
The same evening is both the third day of the thaw and a week since the snow broke.
“Mara found the door on the third day of the thaw”
“a week since the snow broke, and still no word from the pass”
Unless the snow broke well before the thaw began, one of these counts is off.
What it looks for
Details that change
Eye colors, names, addresses, the color of a door.
Your world’s own rules
Magic that costs something, a curse with conditions, a law of the city.
Where people and things are
Someone speaking from two valleys away, a letter that burned last chapter.
Timelines and counts
Ages, dates, how many days have passed, how many guests sat down.
A second pair of eyes
It knows your book
It checks your story against itself, not against the real world. If your moon rises in the west, that’s your rule, and it holds you to it.
It shows its work
Every flag quotes both passages exactly, so you can judge it in a few seconds.
Sure things first
Clear contradictions come first, the maybes after. Short on time? Start at the top.
It never rewrites
It points things out. What to change, and whether to change it, is up to you.
Not instead of your editor or beta readers. Before them, so their notes can go on the story instead of the door.