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Phrase(s): There’s many a slip ’twixt the cup and the lip.
Prov. Many things may happen to prevent you from carrying out what you intend to do. • Bob: Now that I have a contract with a publisher, nothing in the world can stop me from writing this book. Alan: Don’t be so sure. There’s many a slip ’twixt the cup and the lip.
McGrawhill's American Idioms And Phrasal Verbs