The scenario is rather intriguing. You have a group of advanced physics majors (a few being seniors about to graduate) in a honors QM class. You have them a number of questions for a Midterm exam.
Then, after a few weeks, you give them another exam, but using the identical set of questions as the Midterm exam! So what do you expect to happen? It's not what you think, and that is what was tested in this rather intriguing paper.
Abstract: We discuss a case study in which 14 advanced undergraduate physics students taking an honor-level quantum mechanics course were given the same four problems on midterm and final exams. The solutions to the midterm problems were provided to students. Their performance on the final exam shows that although some advanced students performed equally well or improved compared to their performance on the midterm exam on the problems given twice, a comparable number performed less well on the final than on the midterm exam. The wide distribution of students' performance on problems given again suggests that most advanced students do not automatically use their mistakes as an opportunity for learning, repairing, extending, and organizing their knowledge structure. Interviews with a subset of the students revealed attitudes toward problem solving and gave insight into their approach to learning.
The interviews conducted gave a bit of an insight on why a few of the students performed worst in the second exams, and after reading them, it certainly is plausible for that to happen.
I think that for instructors, this might be something to consider, that the ability of a student to do something or answer a question may not reflect on his or her ability to carry that knowledge at a different time. This is not surprising either. I am sure that if someone drops in front of me a QM problem that I used to be able to do when I was an undergraduate student, I won't be surprised if I have regressed and got stuck. There are many instances where our ability to solve or do something is based on how sharp our skill is on that particular area at that given moment. And I haven't looked at an undergraduate QM text in ages. So it is a bit understandable for a few of those students to simply "let go" of some of the stuff they were learning for the Midterm exam when they think it won't come up again.
 A. Mason and C. Singh Am. J. Phys. v78, p.760 (2010).