Fan Casting Identical Twin Mistake in Sort by Tropes

In Real Life, identical twins are still often different enough to be distinguishable. In Fictionland, this isn't always the case. Many times a twin will be mistaken for their sibling, whether or not they realistically would be. This may happen intentionally, as part of a Fake Twin Gambit, a Twin Switch, or if a character is Impersonating the Evil Twin, but just as often it's a complete accident, with hilarity following.