There’s many a slip ’twixt cup and lip
Definition 1 of 1
Pro Tip 1/3Outcome Uncertainty
Reminds that success is never guaranteed until the result is final, expressing caution against premature celebration. 
He may be ahead now, but there’s many a slip ’twixt cup and lip—anything can happen in the final round.
Pro Tip 2/3Expectation versus Reality
Highlights the often unexpected nature of events, emphasizing that reality may not meet our expectations. 
I was sure I'd get the job, but there’s many a slip ’twixt cup and lip—I didn't even make it to the second interview.
Pro Tip 3/3Historical Phrase
Originally from ancient Greek, this phrase has been used in English for centuries, adding a classic and learned tone. 
As we prepared to sign the contract, I was reminded that there’s many a slip ’twixt cup and lip.