千慮 せんりょ いち しつではありませんでした
senryono ichishitsudewārimasendeshita

Noun
Desu Copula

The polite, past negative form of 千慮の一失

1. It was not a mistake of a thousand considerations.; It wasn't due to a lack of thorough thought.

Past/Perfective
Negative
Polite
Formal Register