idiom used to warn not to be too sure of oneself about the result of a plan because things can still go wrong before it is accomplished
THERE'S MANY A SLIP TWIXT CUP AND LIP
Meaning of THERE'S MANY A SLIP TWIXT CUP AND LIP in English
Explanatory English dictionary bed edition. Толковый словарь английского языка Редакция bed. 2012