there is many a slip twixt / between cup and lip

/ðeəɹ ɪz mˈɛni ɐ slˈɪp twˈɪkst ɔː bɪtwˌiːn kˈʌp and lˈɪp/

/ðɛɹ ɪz mˈɛni ɐ slˈɪp twˈɪkst ɔːɹ bɪtwˌiːn kˈʌp ænd lˈɪp/

used to imply that even when something seems certain or likely to happen, there are many opportunities for it to go wrong or for unexpected events to intervene before it is actually achieved