When you 'upset the apple cart' you disturb someone's plans or upset the status quo. When you 'upset the apple cart' you disturb something which was planned or arranged. Similar in meaning to 'rock the boat' or 'to rain on someone's parade'. This is a useful expression you can find used in both social and professional contexts.

1. I don't like upsetting the apple cart, but with all the snow outside I think we should change our plans and not go to the concert this evening.
2. John really gets on my nerves. He likes to be difficult and never misses an opportunity to upset the apple cart. 

1. I think that bringing that issue up at the meeting will only serve to upset the apple cart and cause unnecessary complications.