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You can never be too careful.
Literal
Be-cautious [more-than-に越したこと] is-not.
A classic aphoristic set phrase. ~に越したことはない is a fixed idiomatic frame, literally '[there is] nothing that surpasses X,' meaning 'nothing is better than X,' 'you can't go wrong with X,' 'it's best to X,' 'you can never X enough.' Always used with a dictionary-form verb before に. 越す ('to cross over, to exceed, to surpass') here is in the past-as-relative form 越した. Common collocations: 用心するに越したことはない (can't be too careful), 安いに越したことはない (cheaper is always better). One of the handful of semi-frozen expressions every intermediate learner should memorize.