No one is perfect. A project manager typically does the best job they can,  given the information available at the time. However, project managers sometimes make mistakes. This could be a mistake in communication, in estimation, or in understanding the project "Deliverable", etc. It would have to be a fairly large mistake to be classified as a formal issue, but large mistakes do happen.

Issues management is normally a cold and logical process, involving problem identification and resolution techniques. However, these specific types of issues can be especially difficult to resolve since the project manager may feel some defensiveness (and perhaps embarrassment) for having caused the problem to begin with. If this happens to you while managing a project, use the following steps to deal with it effectively.

1. Own the problem
You must first recognize the problem and own up to the fact that you caused it. If you caused the problem but try to blame it on others, you will probably find that resolving the problem is much more painful to you. If you caused the problem, or if you were partially at fault, be mature and honest enough to own it.

2. Communicate openly
You may be surprised how liberating it can be to just come right out and say that you blew it! I went to a meeting many years ago where the participants were ready to pound on a project manager because of a major problem that he caused. However, the first thing the project manager did when the meeting opened was to admit the mistake and take total responsibility. The rest of the attendees no longer felt like they needed to blame the project manager for all the problems--he had already admitted it! With that quickly out of the way, you can now focus on resolving the problem.

3. Resolve the problem coolly and calmly
You have the personally painful part out of the way. Now look for alternatives and resolve the problem using your normal issues management techniques. Don't get caught back up in the personal pain by acting defensive or by looking for ways that you can save face. Given the mistake made, look for the best resolution for your project.

4. Learn from the mistake
Turn your stumbling block into a stepping stone. Generally, each mistake you make can be turned into a learning experience. You can put better processes in place if that is appropriate. Or you can apply the learning personally and change your management processes (maybe even slightly) so that this type of problem does not occur again.

If you handle problems like this you will generally find that people give you the benefit of the doubt, and, in fact, many will even admire you for the way you address these personal challenges