If a person makes an assumption about something, and they end up making plans based around their expectations despite it not being a guaranteed thing, then the popular expressionDon't Count Your Chickens Before They Hatch is an approriate phrase to use if their assumption turns out to be wrong.
Phrase Origins
According to Idiomation, the origins for this saying might possibly be as old as the 6th century B.C.E. The phrase was used by Aesop, who was a fable writer that lived in ancient Greece.
In one of his fable's, entitled The Milkmad and Her Pail, it tells the story of a young milkmaid who was on her way to the local market in order to sell a pail of milk that she was carrying around on her head. Before even reaching the seller, the milkmaid was already planning on what to do with the money she'd receive, only to accidently drop the pail of milk all over the ground shortly after. She was forced to return home, where her mother shared these words:
"Ah, my child," said the mother, "Do not count your chickens before they are hatched."
"A friend of mine was confident he would win the upcoming marathon and was already planning to celebrate his victory. However, I warned him to not to count his chickens before they hatched."
"I expected to receive my paycheck in the mail today, and went preemptively ordered some expensive items online. It turns out the paycheck won't be arriving for another week, though, so I should have never counted my chickens before they hatched."
Note: For most old phrases, sayings, and other familiar idioms you come across on this site, finding the precise phrase origin is difficult. What's provided is around the earliest known date the phrase or saying was being used in.
Usually the expression comes from a quote found in an old newspaper, book, poem, or play. However, if the idiomatic phrase is already being used in one of those, it's likely already a well known saying, thus it should be assumed the origins are older.
There are rare cases where determining an idiom's origin is possible. Anyways, for the most part, you'll get a rough idea on how long ago certain phrases were being used in, and can get a rough idea on how old they are.
Interested in finding more phrases? The expressions list has more meanings for the sayings and their origins, so try it out!
knowyourphrase.com - Meanings and Origins of Phrases