The politician stumbled over several key facts when he tried to ad-lib, revealing his lack of preparation.

stumble 是动词，本义是"绊倒"，引申义指说话或表演时"结巴、卡壳"，即表达过程中因口误或忘词而出现不流畅，常用搭配 stumble over something 表示"在某个词或某事上卡住"。