Introduction
System design interviews are one of the most challenging parts of the technical interview process at top tech companies like Meta, Google, Amazon, and Netflix. Unlike coding interviews, there is no single "correct" answer — which makes them both exciting and nerve-wracking.
After coaching hundreds of candidates, we have identified the five most common mistakes that even experienced engineers make. Here is how to avoid them.
1. Jumping Straight Into the Solution
The most common mistake is diving into components and databases before understanding what you are building. Interviewers want to see that you can clarify ambiguous requirements.
What to do instead: Spend the first 3-5 minutes asking clarifying questions. Define the scope, identify core features, and agree on constraints with your interviewer.
2. Ignoring Scale Estimates
Many candidates skip back-of-the-envelope calculations entirely. Without understanding the scale, your design choices lack justification.
What to do instead: Estimate the number of users, requests per second, and storage needs. These numbers should directly inform your design decisions (e.g., whether you need caching, sharding, or a CDN).
3. Over-Engineering the Design
Adding Kafka, Redis, Elasticsearch, and a microservices architecture for a system that serves 100 users is a red flag. It signals that you are pattern-matching rather than thinking critically.
What to do instead: Start simple. Add complexity only when your scale estimates justify it. Explain why each component is needed.
4. Neglecting Trade-offs
Every design decision involves trade-offs. Saying "we will use a NoSQL database" without explaining why — or acknowledging what you are giving up — misses the point of the interview.
What to do instead: For every major decision, briefly mention the alternatives you considered and why you chose your approach. This demonstrates depth of understanding.
5. Not Addressing Failure Scenarios
Production systems fail. Interviewers want to know you think about reliability, not just the happy path.
What to do instead: Discuss what happens when a service goes down, a database becomes unavailable, or traffic spikes unexpectedly. Cover retry strategies, circuit breakers, and graceful degradation.
Wrapping Up
System design interviews test your ability to think at scale, communicate clearly, and make well-reasoned trade-offs. By avoiding these five common mistakes, you will stand out from other candidates and demonstrate the engineering maturity that top companies look for.
Ready to practice? Check out our system design course or book a mock interview with a coach who has been through these interviews at FAANG companies.