"Upset the apple cart" is a common phrase that is used to describe a situation where a plan or an arrangement is disrupted or overturned. While the phrase has its roots in the apple industry, it is now used as a figurative expression in everyday conversation. There are several synonyms for this phrase including "throw a spanner in the works", "rock the boat", "derail", and "disrupt". All of these phrases imply that something has gone wrong or that there has been an unexpected turn of events. So if you want to describe a situation where things have gone awry, you can use any of these synonyms for "upset the apple cart".