那是个无心之失。

It was an ___innocent mistake.

那是个无心的评论，我并非有意伤害他的感情。

It was an ___innocent comment; I didn't mean to hurt his feelings.

我一时冲动建议他去试试，结果把一件单纯小事变成了一场噩梦。

My suggestion, made in the moment, turned an ___innocent episode into a nightmare.