A bag of potatoes was discovered in his car.

• A bag of potatoes was discovered in his car. (他的車上被發現一袋馬鈴薯) (正)
• A bag of potatoes were discovered in his car. (誤)

解說：「A bag of + 複數可數名詞」若當句子的主詞，其真正的主詞為 bag，後面須接單數動詞，亦即句子的主動詞須與 bag 一致，而不是跟複數可數名詞一致。介系詞片語「of + 複數可數名詞」係當形容詞用，修飾 bag，對動詞沒有影響。除 a bag of 外，a box of, a pack of 等片語的用法亦同。