the.com/curveball

means An unexpected twist or complication that catches you off guard, something hard to prepare for because it doesn't arrive the way you expected it to.

from Straight from baseball: a pitch thrown with spin so it curves away from its apparent straight path, first named as such in American English in the latter half of the 1800s as pitchers worked out how to make a ball bend through the air. By the early 1900s, 'throw someone a curveball' had already curved itself into general use, meaning to spring something unexpected on a person — the pitch became a metaphor for any trick life throws that breaks late, after you've already committed to swinging.