costly mistake

Translations
дорогостоящая ошибка
The company made a costly mistake by not investing in new technology.
Компания совершила дорогостоящую ошибку, не инвестировав в новые технологии.
Definitions
A costly mistake refers to an error or misjudgment that results in significant financial loss, damage, or negative consequences, often requiring substantial resources to rectify.