by mistake可數嗎

　　例句：

　　I found a mistake in that sentence.

　　我發現那個句子有一個錯誤。

　　You've made several grammatical mistakes in the composition.

　　你的作文中犯了幾處語法錯誤。

　　They made the big mistake of thinking they could seize its border with a relatively small force.

　　他們大錯特錯，居然認為以較少的兵力就能夠奪取其邊境地區。