改错：someone in the office had made a mistake, and the firm ((regretted to have casused))

改错：someone in the office had made a mistake, and the firm ((regretted to have casused))

涉及到regret的用法：
regret doing sth.对某事感到后悔遗憾（已经发生的事）
regret to do sth.为要做的是感到后悔（动作未发生）
regret to have done将来完成时
regret having done:为做过的事感到后悔，抱歉
 
 
综上可见动作已经发生，且发生在过去，应与第四种情况吻合，故错误。
应改成regretted having accused.

regret to do sth.(加动词原形)