there's many a slip
there's many a slip ('twixt cup and lip)many things can go wrong between the start of something and its completion; nothing is certain until it has happened. proverb
there’s ˌmany a ˈslip (’twixt ˌcup and ˈlip)(saying) (of plans, hopes, etc.) nothing is completely certain until it happens because things can easily go wrong: We should get to London before 7 o’clock, but there’s many a slip ’twixt cup and lip. ♢ She aims to get to the top in the company, but there’s many a slip...
The word ’twixt is a short form of the old word betwixt, meaning ‘between’.