From Longman Dictionary of Contemporary Englishupset the apple cartupset the apple cartSPOIL informal to completely spoil someone’s plans → upset
Examples from the Corpus
upset the apple cart• Just when you think you have got your finances whipped into shape something or some one comes along to upset the apple cart.• As I said, you could have upset the apple cart badly.• Our specific role in the political milieu is to upset the apple cart, which is precisely what happened in Florida. 