The lawyer's apparently innocent question was designed to catch the witness off (his) guard.

律师看似不经意的问题是为了使证人措手不及。

《牛津词典》

Just as it looked as though his cards were played out, the wily Mr Chaudhry has caught his opponents off guard by tendering his resignation.

ECONOMIST: Fiji

Gerrard was allowed time and space with the ball in midfield and he used it to telling effect as he slid a ball through to Torres, who took a lovely first touch and held off Gary Caldwell before catching Al-Habsi off guard by taking his shot early to find the bottom corner.

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But midway through the interview, he was caught off guard when his interviewer asked him for his Klout score.

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