Curveball
Etymology
From baseball, where a curveball is a pitch thrown with spin so it breaks away from its apparent path, fooling the batter. To "throw a curveball" moved into general speech for any unexpected complication or trick question. The metaphor keeps the sense of deliberate deception, not mere bad luck.
noun · /KURV-bawl/
Something unexpected and deceptive that arrives when you were braced for the ordinary.
What it names
The surprise designed to wrong-foot you — a move whose whole point is to defy your read.
Example
The interviewer threw a curveball, asking how I would handle being fired.
Related idea
A curveball is thrown to deceive you, whereas a black swan is an unforeseeable shock nobody could have aimed.
See also