那是一个笔误。

It's a slip of the pen.

Source: Tatoeba

Sentence Composition

=1 那是 nàshi
  1. (coll.) of course
  2. naturally
  3. indeed
=1 那是 nàshì
  1. (colloquial) of course
  2. of course
=2 一个 yīge
  1. a; an; one
  2. the whole (afternoon, summer vacation etc)
=2 一个 yīgè
  1. Combination of the numeral 一 (“one”) and the generic classifier 個／个: a; an; one
  2. same
3 笔误 bǐwù a slip of a pen

classifier: 處