GRAMMAR DETAIL

〜としたことが

of all people

Expresses surprise that someone known for being capable or reliable has made an unexpected mistake.

Examples

慎重な君としたことが、署名のない書類を提出するとは珍しい。

It is unlike someone as cautious as you to submit an unsigned document.

真面目な彼としたことが、遅刻するとは。

That someone as conscientious as he is would be late!

冷静な先生としたことが、名前を取り違えるなんて。

For such a composed teacher to mix up the names!

Structure: Attach としたことが to a noun naming a person to express surprise at a mistake unlike their usual competence or behavior.
Basic usage: 真面目な彼としたことが、遅刻するとは。 expresses surprise that someone as conscientious as he is would be late.

Using it about one's own mistake
- It also expresses self-reproach, as in 私としたことが、鍵を忘れた. As when it refers to someone else, it presupposes that the mistake is unlike the person's usual behavior.