“catch sb with pants down”别理解成“撞见某人没穿裤子”，真实含义是撞见...做见不得人的事；出其不意；令...措手不及。

【例句】
Jeff was caught with his pants down when he came to the meeting without having read the brief and the boss started firing questions at him.
杰夫来开会时没看简报，老板开始向他发问令他措手不及。

I think we caught them with their pants down. They're a bit confused.
我觉得我们让他们下不了台了。他们有点不知所措。