upset the applecart

[upset the applecart] or [upset one's applecart] {v. phr.},{informal} To ruin a plan or what is being done, often by surprise oraccident; change how things are or are being done, often unexpectedly;ruin or mix up another person's success or plan for success.

Johnupset the other team's applecart by hitting a home run in the lastinning and we won the game.

We are planning a surprise party forBill, so don't let Mary upset the applecart by telling him before theparty.

Frank thinks he is going to be the boss, but I'll upset hisapplecart the first chance I get.

Compare: ROCK THE BOAT.