Upset the apple cart is an Idiom, which means to spoil a plan that was well organised or spoil someone's progress. Apples are so beautifully placed in a cart which needs much time for the arrangement. Nevertheless, if the cart goes upside down, the apples all go ransacked, and makes a mess. From this everyday incident, the idiom Upset the apple cart has originated.
When you have planned to go for a vacation, booked your tickets well in advance, but unexpectedly due to heavy rains if the train or flight gets cancelled, you may say the cancellation has upset the apple cart.
The team leader told his team members that the timely submission of the project was very vital and asked them not to upset the apple cart by wasting their time in arguments.
My friend had borrowed my notes and lost it. My examination is round the corner, and she has upset my apple cart.