There is many a slip between a cup and a lip

Origin
This is a very ancient proverb originating from south-west part of the Asian continent. It is first found in Robert Burton’s The Anatomy of Melancholy (1621-1651).
Explanation
It implies that even when the outcome of an event seems certain, things can still go wrong. It means that there is many a time-gap between the occurrence of two events and anything can happen in this time gap and things can change in a second. We should never be sure of our success in our life until we have achieved it and should work for it until the last second. It generally refers to a situation where things did not happen at the last minute the way they were expected due to unforseen reasons. It is something that you say in order to warn someone not to be too confident about the result of a plan, because many things can go wrong before they are completed. Many things can go wrong between the start of a project and its completion; nothing is certain until it has happened.
Examples
I was to get a raise this year, but it didn’t happen due to the merger.
Well my friend, there is many a slip between a cup and a lip.
We still might finish in time for the deadline, but there’s many a slip between a cup and a lip.