grosse erreur, bévue
Thanks to a bureaucratic blunder, we don't have access to the building anymore.
À cause d’une erreur administrative, nous n’avons plus accès au bâtiment.
It's worse than a crime; it's a blunder.
C’est pire qu’un crime : c’est une bévue.
How could I have made such a blunder?
Comment ai-je pu commettre une telle erreur ?
— The Irony of Fate, or Enjoy Your Bath! (1976)
As a result of his carelessness, he made a serious blunder.
Par sa négligence, il a commis une grave erreur.