1. something unexpected that causes confusion or disruption
throw a curveball and upset the apple cart
A 'curveball' is a noun meaning something which is unexpected, unplanned for and disruptive usually 'a curveball' refers to something negative which disrupts a plan. You can 'throw a curveball' or 'encounter a curveball'. This phrase is similar in meaning to 'throw a spanner in the works."