There's many a slip 'twixt the cup and the lip
Refers to how easily things can go wrong or be lost at the last moment. It means that you should be cautious and not count your chickens before they hatch. Even when success seems certain, something can still go wrong and prevent it from happening. It highlights that until an outcome is fully realized, there's always a chance for things to deviate from the expected path.
The origin of the proverb is ancient, with variants found in Greek and Latin texts dating back to the 2nd century AD according to BookBrowse.com. One popular origin story involves a Greek myth about King Ancaeus, who, despite returning safely from a voyage, was killed by a boar while rushing to drink wine from his vineyard, thus proving a prophecy true. An English translation of Erasmus's 1523 work by Richard Taverner in 1539 rendered the proverb as "Many thynges fall betwene the cuppe and the mouth ... Betwene the cuppe and the lyppes maye come many casualties".
The modern form of the English proverb where the word slip is used to rhyme with lip developed in the late 18th and early 19th century. For example, an early record is in Catharine Maria Sedgwick's The Linwoods: or, "Sixty Years Since" in America (1835), in the form "there is many a slip between the cup and the lip".
