there's many a slip twixt cup and lip

The Dutch humanist Erasmus (c. 1466 – 1536), in his collection of proverbs called Adagia, noted that the Carthaginian grammarian Sulpicius Apollinaris (fl. 2nd century C.E.) recorded two proverbs, one in Greek and the other in Latin, with the same meaning: πολλὰ μεταξὺ πέλει κύλικος καὶ χείλεος ἄκρου (pollà metaxù pélei kúlikos kaì kheíleos ákrou, literally “much takes place between the (wine) cup and the upper lip”) and multa cadunt inter calice[m], supremaq[ue] labra (literally “many things fall between the chalice, and the upper lips”). The earliest English version of the expression recorded by the Oxford English Dictionary is Richard Taverner’s 1539 translation of Erasmus’s work: see the quotation. The proverb refers to the possibility of a drink being spilled from a cup while it is being raised to the lips and before it can be drunk.

来源：wiktionary