Case Interview Mistakes: Six a Firm Publishes Itself

Six case interview mistakes a firm publishes under its own name

The list sits on BCG's case preparation page at careers.bcg.com, beside six recommendations: Listen Actively, Be Yourself, Think Structurally, Communicate Clearly, Clarify Assumptions, Show Your Thinking.

Pair the pitfalls by direction and the list stops looking like a warning and starts looking like a diagnosis.

Two of those six cannot both be fixed by speaking more, and two cannot both be fixed by speaking less. The firm is describing a band with an edge on each side, which means the useful question is whether you are inside it rather than which direction to push.

The same page states the standard against which all six are faults: "What makes case study interviews different is that there isn't always a single 'right' answer". A candidate hunting for one answer produces at least three of the six. They rush, they stay silent while calculating, and they ignore a nudge that would have pointed elsewhere.

Three myths one firm corrects in print

Accenture's case workbook, at accenture.com, lists three beliefs it calls myths and prints the opposing fact beside each.

1. That there is one right answer. The published fact: "Usually there are several credible approaches and solutions".
2. That the interviewer is waiting for the right answer. The published fact: "The interviewer expects a thoughtful, structured response".
3. That industry knowledge is required. The published fact: "The interviewer expects you to be effective with the knowledge you have and make hypotheses based on sound common sense".

The third one is the expensive myth, because it causes a specific behavior. A candidate who believes they need industry knowledge and does not have it goes quiet, or hedges, or asks for facts instead of making an assumption. All three of those are on BCG's pitfall list.

The same workbook states what success does depend on: "how clearly you define the problem, how logically you structure your analysis, your quantitative analysis skills and how well you communicate your thoughts to the interviewer." Four things, none of which is a fact about the industry.

The document carries a 2018 copyright and reports a headcount of 435,000 against 791,000 in the firm's September 2026 job postings. It is dated, and we say so rather than presenting it as current.

The unit mistake in a case interview, and the factor it costs

Arithmetic mistakes in a case are recoverable. Unit mistakes are expensive, because they multiply rather than add, and one firm's own blog supplies the example.

On L.E.K.'s careers blog at lek.com, a first-year London associate describes sizing the wedding market as if each person married separately rather than two people per wedding. The interviewer credited his method and pointed out the error. That is one person describing his own experience on his employer's platform, and it is not a statement about how the firm grades.

The factor there was two. Here is one where the factor is 2.5, with stated assumptions.

A product is bought by the household rather than by the person. United States population is taken as 330 million, average household size as 2.5 people, so households are 132 million. Each household replaces the product once every four years.

Recomputed by hand on 30 September 2026. 330 million over 2.5 is 132 million households. 132 million over 4 is 33 million units a year. The wrong basis gives 330 million over 4, which is 82.5 million, and 82.5 over 33 is exactly 2.5.

The check that prevents it takes four seconds and one sentence: say the unit out loud before you multiply. Say households where the buyer is a household. Say per year where the cycle is four years. Any sizing where the two differ is a sizing where this error is available.

Three other arithmetic mistakes are worth naming because they are about the sentence rather than the sum.

• Quoting a percentage without its base. Two points of margin on revenue can be 38 percent of profit. Both are true and they are different statements.
• Calling a fraction by the wrong name. 8.6 out of 14 is 61.4 percent. Two thirds is 66.7 percent. The gap is 5.2 points and it can flip a recommendation.
• Rounding silently. Saying five when the calculation gives 4.8 is fine if you say roughly. Saying five flat is an error.

A candidate who names his own case interview mistake

Firms publish what they want. Almost nobody publishes what it looked like to fail. One account exists in our corpus.

A trade publication interview at consultancy.uk carries a consultant describing his own applications after an MBA. On one of them he says: "I did not move forward after the second interview at A.T. Kearney in September 2016. I realised my approach in solving their interview case was a little too entrepreneurial and lacked the structure A.T. Kearney was looking for." He adds what he changed: "I decided to spend more time in refining my structure and practicing more cases with fellow students."

Read that carefully for what it is and is not. It is one candidate, one office, one year, told six months after the fact, and it is his reading of his own rejection. Nobody at the firm said that was the reason. The firm was renamed Kearney in 2020.

What makes it worth quoting is the shape of the self-diagnosis. He does not say he got the arithmetic wrong or ran out of time. He says the approach was creative and unstructured, which is a failure mode that feels like a strength in the room. A candidate generating ideas quickly experiences that as going well.

The uncorroborated status matters, so we hold it at that level: a single account of one person's experience. It is no evidence at all about how the firm assesses.

The framework mistake in case interviews, and a firm's warning

The most argued-about mistake in this subject has a firm on each side, which is why treating either position as a rule is wrong.

Kearney, in a Wayback snapshot at kearney.com, writes: "Try to avoid forcing your solution to fit a standard framework. The only things it needs to fit are the problem, the business, and the objectives."

A recruiting lead at Simon-Kucher's Frankfurt office writes the opposite emphasis: "A solid understanding of economic principles and frameworks can also be beneficial, especially in our core focus areas such as pricing, marketing, digitalization, and sales."

Both are published. Our reading, labeled as ours: knowing frameworks is an asset and reaching for one before you have read the prompt is the mistake. The two statements are compatible under that reading, and neither firm endorses it.

The tell for a forced framework is mechanical. Look at your branches and ask whether any of them could be deleted without leaving a hole. A four-box structure where one box has nothing under it is a template that came before the problem. Kearney's line about fitting the business is the test: swap the client for a different industry and see whether your branches change.

A second frequently-cited mistake belongs here too, and it is published as a caution rather than a ban. Deloitte writes: "Ask questions, but don't use them as a crutch. While it's important to ask clarifying questions, avoid relying on them too heavily. We're interested in how you connect the dots when solving the problem." Asking is fine. Asking instead of deciding is the mistake.

Where each mistake lands in our weights

The four lines below are ours, read from the grading code in src/lib/grading. No firm in our corpus publishes a marking scheme.

A pre-interview error list you can actually use

Everything above compresses into eight checks. All eight are attributable to a published source or to our own arithmetic.

1. Am I inside the speaking band? Narrate while computing and let the interviewer finish. Two of BCG's six.
2. Did I open every branch before going deep? Coverage is the expensive line.
3. Would my structure change for a different industry? Kearney's test for a forced framework.
4. Did I say the unit before multiplying? Households, years, per visit.
5. Did I say the base with every percentage? Of revenue, of profit, of the gap.
6. Did I announce every rounding? Roughly, about, call it.
7. Did I act on the last thing the interviewer said? Ignore Feedback is on the published list.
8. Did I assume rather than ask, where asking would stall? Accenture's third myth is what stops candidates from assuming.

Our structure drill names the first correction on Coverage, Separation, Depth and Concision, and our math drill reports Units separately so a basis error shows up on its own line. Both are free and neither needs an account. Neither can catch a silence or an interruption, which is why the two speaking-rate pitfalls stay on the list you work with a person.

For the recovery once a mistake has happened, see recovering mid-case. For the framework question in more depth, see when to use a framework. For the arithmetic habits behind checks four through six, see case interview math tips.