There's many a slip 'twixt the cup and the lip

there's many a slip twixt cup and lip

proverb Even something that one feels confident will succeed can have disastrous problems before it concludes. "Twixt" is a shortening of "betwixt," an archaic form of "between." Everything seems to be going smoothly, but there's many a slip twixt cup and lip, so don't lose focus until we're over the finish line.
See also: and, cup, lip, many, slip, twixt
Farlex Dictionary of Idioms.

There's many a slip 'twixt the cup and the lip

. Prov. Many things may happen to prevent you from carrying out what you intend to do. Bob: Now that I have a contract with a publisher, nothing in the world can stop me from writing this book. Alan: Don't be so sure. There's many a slip 'twixt the cup and the lip.
See also: and, cup, lip, many, slip
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

many a slip 'twixt the cup and the lip, there's

Nothing is certain until one possesses it. This old proverb is thought by many to come from the ancient Greek legend of Antaeus, helmsman of the ship Argo. A slave told him he would never live to taste the wine from his own vineyard. As some wine from his grapes was set before him, he sent for the slave to show him his mistake, but the slave allegedly said, “There’s many a slip ’twixt the cup and the lip.” Just then a messenger arrived, telling him the Calydonian boar was destroying his vineyard. Antaeus jumped up, set down his wine, and went out to kill the boar, but was himself killed by the ferocious animal. Another writer believes the phrase comes from Homer’s Odyssey, in which Odysseus aimed an arrow at Antinous as he was about to drink some wine. The arrow hit him in the throat, and the cup fell from his hands before he could drink.
See also: and, cup, many, slip
The Dictionary of Clichés by Christine Ammer
See also:
  • there is many a slip twixt cup and lip
  • there's many a slip
  • there's many a slip twixt cup and lip
  • twixt
  • village
  • it takes a village
  • teach a man to fish
  • a little knowledge is a dangerous thing
  • a little learning is a dangerous thing
  • the best-laid plans of mice and men
References in periodicals archive
There's many a slip 'twixt the cup and the lip. So another couple that really wants the house could submit a written offer that's "subject to the nonperformance of the existing sales contract." Sellers will usually accept such a backup proposal -- it's like having a belt and suspenders to hold your pants up.
Hard to fool assessor with artificially low home sale price
"Nevertheless, we should recall the old proverb that "there's many a slip 'twixt the cup and the lip," he then added.
It's too early for Lebanon to celebrate gas wealth: study
There's many a slip 'twixt the cup and the lip, goes the old English proverb.
Hold the cheers