There's many a slip 'twixt the cup and the lip is an old English proverb

Proverb

A proverb is a simple and concrete saying popularly known and repeated, which expresses a truth, based on common sense or the practical experience of humanity. They are often metaphorical. A proverb that describes a basic rule of conduct may also be known as a maxim...

. 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

Adagia

Adagia is an annotated collection of Greek and Latin proverbs, compiled during the Renaissance by Dutch humanist Desiderius Erasmus Roterodamus. Erasmus' collection of proverbs is "one of the most monumental ... ever assembled" Adagia (adagium is the singular form and adagia is the plural) is an...

," I.iv.1 ("Multa cadunt inter calicem supremaque labra") which appears to derive from an epigram by Palladas

Palladas

Palladas was a Greek poet, who lived in Alexandria, Egypt. All that is known about this poet has been deduced from his 151 epigrams preserved in the Greek Anthology....

 in "The Greek Anthology" (X, 32).

The proverb supposedly comes from a Greek

Greeks

The Greeks, also known as the Hellenes , are a nation and ethnic group native to Greece, Cyprus and neighboring regions. They also form a significant diaspora, with Greek communities established around the world....

 legend in which one of the Argonauts

Argonauts

The Argonauts ) were a band of heroes in Greek mythology who, in the years before the Trojan War, accompanied Jason to Colchis in his quest to find the Golden Fleece. Their name comes from their ship, the Argo, which was named after its builder, Argus. "Argonauts", therefore, literally means...

 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.

The first occurrence of the proverb in English, according to the Oxford English Dictionary

Oxford English Dictionary

The Oxford English Dictionary , published by the Oxford University Press, is the self-styled premier dictionary of the English language. Two fully bound print editions of the OED have been published under its current name, in 1928 and 1989. The first edition was published in twelve volumes , and...

, is in Thackeray

Thackeray

Thackeray is the name of:*William Makepeace Thackeray, a novelist*Bal Thackeray, an Indian politician*Edward Talbot Thackeray, a recipient of the Victoria Cross*A David Thackeray, a South African astronomer...

's Pendennis

Pendennis

Pendennis is a novel by the English author William Makepeace Thackeray. It is set in 19th century England, particularly in London. The main hero is a young English gentleman Arthur Pendennis who is born in the country and sets out for London to seek his place in life and society...

, 1850.