A posting I came across
“I am just curious as to whether or not any of you have recovered from a bad interview. For instance, you talked too much or weren’t focused in your responses .. maybe you were distracted from something that happened just prior to your interview and it threw you off your game or you didn’t feel as though you connected with the interviewer or didn’t say something you thought would have been great to bring up.
Did you make up for it in your Thank you letter or did you do a follow up call and as a result, you got another chance?”
Review very carefully in your mind whether you actually made mistakes in the interview, or simply didn’t perform as well as you could. For example, if communication skills are critical to the job, and you garbled what you said in significant sections, then you need to address that in your thank you letter. Make a very simple statement explaining it, but don’t go into much detail - the more you talk about it, the more you emphasize the negative. Then immediately go into accomplishments and results that demonstrate that you actually do have powerful communication skills.
On the other hand, if you simply didn’t perform as well as you hoped, or neglected to include a story to back up an important skill / area of experience, then there’s no need to talk about your performance in the interview. Simply provide the proof in your thank you letter of the qualifications and results that make you a powerful candidate, including the bullet points to prove any missing points. You could even preface something that you omitted to mention in the interview, like “I may not have mentioned…”
The Thank You letter is really a marketing document, just as is the Cover Letter. You need to make your most powerful arguments in both…and in the Thank You you have the advantage of already having had the deep discussion with the interviewers, so that you can tailor it much more effectively to their challenges.
Here’s an article on the topic: