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 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.