He might say something without meaning to, which would do me great harm.

to为不定式符号，后面省略了动词say it, 等于without meaning to say it.

He might say something but didn't mean to say it. 他可能会说一些不是存心要说的话，即他说的话不是他本来想要表达的意思。

当一个不定式的动作在前面已经提及，则不定式可以省略，只保留不定式符号。例如：

I could have helped him, but I didn't want to.