What causes the “A variable mismatch has been detected” error in WordPress?

Solution:

The WordPress error:

A variable mismatch has been detected.
Sorry, you are not allowed to view this item.

elseif ( isset( $_GET[ $wpvar ] ) && isset( $_POST[ $wpvar ] ) && $_GET[ $wpvar ] !== $_POST[ $wpvar ] ) {
    wp_die(
        __( 'A variable mismatch has been detected.' ),
        __( 'Sorry, you are not allowed to view this item.' ),
        400
    );
}

✅ Explanation:

WordPress expects that if a variable exists in both $_GET and $_POST, the values must be identical.

If they differ, WordPress throws this error to prevent unexpected behavior or potential security issues.

In your case:

$_GET[‘page’] did not equal $_POST[‘page’], which triggered the error.

Fix:

Ensure that any variables sent via both GET and POST requests have the same value.

Avoid sending conflicting values for the same key in forms and URLs.