。
She must have caught the wrong bus.
Literal
She [topic-は] bus [object-を] mistook [must be-にちがいない].
~に違いない ('must be, there's no doubt') is one of Japanese's certainty-markers. It's stronger than ~だろう ('probably') and ~かもしれない ('might'); the speaker is essentially certain. The construction attaches to verbs in their plain form, nouns directly, or adjectives — here it follows the past form 間違えた. 間違える ('to mistake, get wrong') is a transitive verb taking a direct object — バスを間違える ('get the wrong bus'), 道を間違える ('take the wrong path'), 答えを間違える ('get the answer wrong').