[upset the applecart] or [upset one's applecart] {v. phr.}, {informal} To ruin a plan or what is being done, often by surprise or accident; change how things are or are being done, often unexpectedly; ruin or mix up another person's success or plan for success.
* /John upset the other team's applecart by hitting a home run in the last inning and we won the game./* /We are planning a surprise party for Bill, so don't let Mary upset the applecart by telling him before the party./* /Frank thinks he is going to be the boss, but I'll upset his applecart the first chance I get./
Compare: ROCK THE BOAT.