There's many a slip 'twixt the cup and the lip is an old English proverb. It implies that even when the outcome of an event seems certain, things can still go wrong.
A Latin form is found in Erasmus's "Adagia," I.iv.1 ("Multa cadunt inter calicem supremaque labra") which appears to derive from an epigram by Palladas in "The Greek Anthology" (X, 32).
The proverb supposedly comes from a Greek legend in which one of the Argonauts returns home to his winery. A local soothsayer had previously predicted the Argonaut would die before he tasted another drop of his wine, thus the Argonaut calls the soothsayer and toasts him for the Argonaut had survived his journey. The soothsayer replies to the toast with a phrase corresponding to the English proverb. As he finishes his toast, the Argonaut raises a cup filled with wine to his lips but is called away to hunt a wild boar before he could take a sip. The Argonaut is killed hunting the boar.[1]
The first occurrence of the proverb in English, according to the Oxford English Dictionary, is in Thackeray's Pendennis, 1850.[2]