Catch sb off (their) guard:趁某人毫无防备之时 to happen when sb is not prepared
The question caught him off his guard and he couldn't answer.
他对这个问题毫无准备,一时哑口.
Businesses were caught off guard by the sudden rise in interest rates.
利率突然上升使各行各业措手不及.

Catch sb red-handed:当场抓住某人(做坏事或犯罪) to find sb while they are doing sth wrong, committing a crime
The thief was caught red-handed as she was emptying the till.
小偷正准备倒钱的抽屉时被当场捉获.