"Curveball" is a term used to describe something unexpected or surprising that happens, similar to a plot twist or a surprise in a story. It refers to a sudden change or a surprising event that catches someone off guard.
He didn't let Darren throw curveballs in Little League. (si.com)
Features In addition to the new look, Lenovo throws a couple of unexpected curveballs in terms of the port selection, like a power connector that doubles as a USB port. (pcmag.com)
Someone is counting the percentage of curveballs thrown by every pitcher, and measuring how each batter performs when his team is down three or more runs in the seventh inning or later. (moneysense.ca)