Upset the applecart
Spoil carefully laid plans, as in: Now don't upset the applecart by revealing where we're going. This expression started out as upset the cart, used since Roman times to mean "spoil everything." The precise idiom dates from the late 1700s.
The use of a cart piled high with apples as a metaphor for a satisfactory but possibly precarious state of affairs is recorded in various expressions from the late 18th century onwards.
If someone or something upsets the applecart, they do something which causes trouble or which spoils a satisfactory situation. e.g. It will only upset the applecart and confuse the issue if the topic is raised too soon.
Note: You can also say that someone or something overturns the applecart. She still has the power to overturn the applecart by the sheer force of her personality and vocabulary.
