Rule reminder
The congiuntivo passato (present perfect subjunctive) is formed with: present subjunctive of avere/essere + past participle. It is used in subjunctive contexts when the action in the subordinate clause happened BEFORE the action of the main clause (which is in the present tense). Example: 'Spero che tu abbia capito' — I hope you understood (before now).
It is strange that no one noticed the error. Choose the correct form: 'È strano che nessuno _____ l'errore.'