Robin's answer is a masterclass in how not to learn from mistakes. The moment people realize that there is blame being handed out they will obfuscate and hide their mistakes as best they can, scapegoating others and generally making it impossible to learn anything useful.
A much better idea is to do a no-blame post-mortem of the project. The goal is to go over the entire project from start to finish, examining at each stage what worked well and what went wrong. At no point should any blame be assigned to any member of the team. Instead the focus should be on identifying failures and putting procedures in place to make sure they don't happen again.
To err is human which is why we have checks and procedures in place. Therefore if a mistake goes uncorrected or unnoticed it is a problem with the procedures, not the individual (except in extreme circumstances such as gross negligence or misconduct).
In fact having a no-blame culture is a great way to prevent these things happening in the first place because people will feel able to come forward with issues when they happen, not long after they have caused a huge problem.
