10 Common Technical Interview Mistakes (And How to Avoid Them)

After reviewing thousands of interview performances, clear patterns emerge. Most candidates fail not because they lack skills, but because they make avoidable mistakes. Here are the top 10—and how to avoid them.

Mistake #1: Starting to Code Immediately

**What happens**: The interviewer presents a problem, and the candidate immediately starts typing code.

**Why it's bad**: You often solve the wrong problem or miss key constraints. You also lose the chance to show your thought process.

**The fix**: Take 2-3 minutes to:

1. Repeat the problem in your own words

2. Ask clarifying questions

3. Work through an example by hand

4. Outline your approach before coding

Mistake #2: Staying Silent

**What happens**: The candidate thinks silently for minutes at a time, leaving the interviewer guessing.

**Why it's bad**: The interviewer can't evaluate your problem-solving process. They might think you're stuck when you're actually making progress.

**The fix**: Narrate your thinking:

- "I'm considering two approaches..."

- "Let me think about the edge cases..."

- "I'm wondering if a hash map would help here..."

Mistake #3: Ignoring Time Complexity

**What happens**: The candidate submits a working solution without discussing or optimizing its efficiency.

**Why it's bad**: A brute-force solution isn't impressive. Interviewers want to see you can identify and implement efficient algorithms.

**The fix**: Always:

1. State the time and space complexity of your solution

2. Ask if optimization is needed

3. Know common optimizations (hash maps, binary search, two pointers)

# Don't stop here:
def contains_duplicate(nums):  # O(n²)
    for i in range(len(nums)):
        for j in range(i + 1, len(nums)):
            if nums[i] == nums[j]:
                return True
    return False

# Optimize to this:
def contains_duplicate(nums):  # O(n)
    return len(nums) != len(set(nums))

Mistake #4: Poor Variable Names

**What happens**: Code is filled with single-letter variables like a, b, x, i, j.

**Why it's bad**: Code becomes hard to follow, increasing the chance of bugs and making it harder for the interviewer to evaluate.

**The fix**: Use descriptive names:

# Bad
def f(a, t):
    d = {}
    for i, x in enumerate(a):
        c = t - x
        if c in d:
            return [d[c], i]
        d[x] = i

# Good
def two_sum(nums, target):
    index_map = {}
    for i, num in enumerate(nums):
        complement = target - num
        if complement in index_map:
            return [index_map[complement], i]
        index_map[num] = i

Mistake #5: Not Testing Your Code

**What happens**: The candidate says "I think that's it" without walking through the code.

**Why it's bad**: Bugs slip through. You miss easy opportunities to catch errors.

**The fix**: Walk through your code with a simple example:

- Trace the values of key variables

- Check boundary conditions

- Verify the output matches expected results

Mistake #6: Getting Defensive About Feedback

**What happens**: When the interviewer suggests a correction or alternative, the candidate pushes back or becomes defensive.

**Why it's bad**: It suggests you're difficult to work with. Collaboration is a key evaluation criterion.

**The fix**: Welcome feedback gracefully:

- "That's a great point, let me adjust..."

- "Ah, you're right, I missed that edge case..."

- "Good catch! Here's how I'd fix it..."

Mistake #7: Freezing When Stuck

**What happens**: The candidate hits a wall and sits in uncomfortable silence, unsure how to proceed.

**Why it's bad**: It wastes precious interview time and increases anxiety.

**The fix**: Have a recovery strategy:

1. Verbalize where you're stuck: "I'm not sure how to handle the case where..."

2. Try a different approach: "Let me think about this differently..."

3. Ask for a hint: "Could you point me in a direction?"

Mistake #8: Over-Engineering the Solution

**What happens**: The candidate builds an elaborate solution with unnecessary abstractions, design patterns, or optimization.

**Why it's bad**: You waste time and increase complexity. The interviewer wants working code, not a framework.

**The fix**: Start simple. Get a working solution first, then optimize if asked. "KISS" (Keep It Simple, Stupid) applies here.

Mistake #9: Not Handling Edge Cases

**What happens**: The code works for the happy path but fails on edge cases.

**Why it's bad**: Edge case handling separates good engineers from great ones.

**The fix**: Always consider:

- Empty input

- Single element

- Duplicates

- Negative numbers

- Maximum/minimum values

- Null/undefined values

def find_max(nums):
    # Don't forget edge cases!
    if not nums:
        return None  # or raise an exception
    
    max_val = nums[0]
    for num in nums[1:]:
        if num > max_val:
            max_val = num
    return max_val

Mistake #10: Poor Time Management

**What happens**: The candidate spends 30 minutes on the first problem and rushes (or doesn't finish) the second.

**Why it's bad**: Incomplete solutions are harder to evaluate positively.

**The fix**:

- Know typical time allocations (10 min understand, 20 min code, 10 min test/optimize for a 45-min interview)

- If stuck on optimization, get a working solution first

- Keep an eye on the clock

Summary Checklist

Before your next interview, review this checklist:

- [ ] Clarify the problem before coding

- [ ] Think out loud throughout

- [ ] Analyze time/space complexity

- [ ] Use descriptive variable names

- [ ] Test your code with examples

- [ ] Accept feedback gracefully

- [ ] Have a strategy for when you're stuck

- [ ] Start simple, optimize later

- [ ] Handle edge cases

- [ ] Manage your time wisely

Practice Makes Perfect

The best way to avoid these mistakes is deliberate practice. [AlgoArena's battle mode](/lobby) simulates real interview pressure, helping you develop good habits under time constraints.

Every mistake you make in practice is one you won't make in your actual interview.

**Start practicing today**: [Practice Problems](/practice) | [Battle Mode](/lobby)