Meredith Lavine, Artfulresumes.com:
For starters, never say that you don’t have a weakness or that you can’t think of one. This is received poorly by interviewers. Everyone has weaknesses. Saying that you are a hard worker or a perfectionist are also trite. The other suggestion is to make sure you don’t select a weakness that can be attributed to your character or work ethic or affect your performance for that particular job.
Select a technical weakness that you are making strides with and the steps you are actively taking to fix it. Technical weaknesses are the easiest to fix. For example, “A weakness for me right now is getting up to speed on MS Project, and I am taking step x, y and z to flatten the learning curve so I can enhance my skills as a project manager.” Or, “Sometimes, I get nervous presenting in public, so I am taking a Presentation Skills course and have joined Toastmasters to address it.” Always follow up the weakness with the positive steps that you are taking to correct it and strive for self-improvement. Be positive presenting the steps you have taken to fix it.
CAUTION: Do not select a weakness that is a competency of the position for which you are interviewing!
Regarding professional “greatest disappointments”, select one that was more or less beyond your control and where you can demonstrate the steps that you took to get the situation back on track. Be brief and succinct, don’t ramble. Steer clear of negative language and show that you have accepted and learned from it.