The common pitfalls are
- to memorize Case In Point or any other preparation guide.
- to try to be too creative and confuse the interviewer completely
- fail to justify your arguments and ideas with the evidence presented to you
- to forget how to crunch and apply numbers to substantiate your claims
- to forget to apply a "sanity check" on your solutions and recommendations
My advice is to practice around twenty cases to become familiar and comfortable with the process.
Beyond that, when you are in the interviewing room and need to separate yourself from an already accomplished set of students, you will need to think outside of the box while delivering your thoughts in a linear, articulate, and professional manner.
Remember that the cases given to you were actual problems the consultants have solved over the past few months or even year. They know what were terrible approaches, average approaches, and exceptional ways to solve the problem.
To "crack the case" means that you will go through a similar process of research, analysis, and delivery as the consultant did in his actual work.
Always drive yourself to get to the final answer - and directly nail the question the interviewer posed at the very beginning. Don't get sidetracked if you miss a sub-part of the question. The point is to stay focused and explore the most high impact avenues to arrive at the final answer.
Remember to think about the ramifications of the question that was asked -- this is what interviewers think as being "inside the box", as parts of the process of solving a problem is duplicable from one profit cutting case to the next, from one merger strategy to the next, and so forth.
However applying one way of thinking to every problem will not get you the job. As I said, don't memorize frameworks and force fit them into a problem you get. This is the number one pet peeve interviewers have.
The most successful interviewees demonstrate that they can "think outside of the box" -- they come up with client recommendations that were better, meaning more original, feasible, and client-friendly, than the actual ones that were delivered by professional consultants.
There was only one summer business analyst hired to work at McKinsey from Wharton last year. There are literally only a few slots open from each school for consultant interns at the other top management consulting firms. Some years, zero interns are hired from some top target schools.
The way to stand out is to not only nail the case but to kill it. Doing that requires original thinking and getting the basics right.