❌ Bad: "Let's try adding a timeout here"
✅ Good: "The timeout is occurring because X. Here's proof: [evidence]"

1. Reproduce the issue reliably
   - Minimal reproduction case
   - Consistent reproduction steps
   - Document environment/conditions

2. Gather evidence
   - Error messages (full stack traces)
   - Logs (with timestamps and context)
   - State at failure point
   - Input data that triggers issue

3. Form hypothesis
   - Based on evidence, not guesswork
   - Specific and testable
   - Includes mechanism of failure

Bug: User authentication fails intermittently

Investigation:
1. Reproduced: Fails every ~10th login attempt
2. Evidence: 
   - Error: "Invalid token signature"
   - Logs show token created at 14:32:15, validated at 14:32:17
   - Server logs show time drift between auth & API servers
3. Hypothesis: Clock skew causing token validation failures
   - Auth server: 14:32:15
   - API server: 14:32:10 (5 seconds behind)
   - Token "not yet valid" due to nbf (not before) claim

1. Check for similar issues
   - Same error in other places?
   - Same pattern in related code?
   - Recurring in error logs?

2. Identify scope
   - One function or entire subsystem?
   - One user or all users?
   - One environment or all?

3. Find common factors
   - Timing (time of day, duration)?
   - Data characteristics?
   - Execution path?

1. Design tests that prove/disprove hypothesis
2. Add instrumentation if needed
3. Run experiments systematically
4. Document results

// Hypothesis: Clock skew causes token failures

// Test 1: Artificially set server clocks in sync
// Result: No failures in 100 attempts ✓

// Test 2: Increase token nbf tolerance to 10 seconds  
// Result: No failures in 100 attempts ✓

// Test 3: Log exact time delta when failures occur
// Result: All failures show 4-6 second clock difference ✓

// Conclusion: Hypothesis confirmed

1. Address root cause
   - Fix underlying issue
   - Not just surface symptoms
   
2. Add safeguards
   - Validation
   - Error handling
   - Monitoring

3. Verify fix
   - Reproducer no longer triggers issue
   - Related edge cases handled
   - No new issues introduced

# Git bisect example
git bisect start
git bisect bad HEAD        # Current broken state
git bisect good v1.2.0     # Last known working

# Git will checkout commits for testing
# Test each: git bisect good / git bisect bad
# Automatically finds breaking commit

Working environment:
- Node 18.16.0
- Dependency A v2.1.0
- Feature flag X: off

Broken environment:  
- Node 18.17.0  ← Suspect
- Dependency A v2.1.0
- Feature flag X: off

Test: Change Node version → Bug disappears → Root cause found

// Not enough information
function processUser(user) {
  const result = complexOperation(user);
  return result; // Fails sometimes, why?
}

// Rich instrumentation
function processUser(user) {
  logger.debug('processUser start', { 
    userId: user.id, 
    userState: user.state,
    timestamp: Date.now() 
  });
  
  const result = complexOperation(user);
  
  logger.debug('processUser complete', { 
    userId: user.id, 
    resultStatus: result.status,
    duration: Date.now() - start 
  });
  
  return result;
}

"When a user clicks login, we:
1. Hash their password ← Wait, are we using the same salt?
2. Compare to database
3. ... oh. We changed the salt algorithm last week."

Modern debuggers (rr, WinDbg, Chrome DevTools) can:
- Record execution
- Replay backwards
- Find exact moment state became invalid

// Symptom: Intermittent failures, works in debugger
// Root cause: Async operations completing in wrong order

// Bad
let userData = null;
fetchUser().then(data => userData = data); // Async
sendEmail(userData.email); // Runs before fetch completes! ❌

// Fixed  
const userData = await fetchUser();
sendEmail(userData.email); // ✓

// Symptom: Tests pass individually, fail together
// Root cause: Tests sharing state

// Bad - shared state
const cache = {}; // Global
test('test1', () => { cache.foo = 'bar'; });
test('test2', () => { expect(cache.foo).toBeUndefined(); }); // Fails! ❌

// Fixed - isolated state
test('test1', () => { 
  const cache = {}; 
  cache.foo = 'bar'; 
});
test('test2', () => { 
  const cache = {}; 
  expect(cache.foo).toBeUndefined(); 
}); // ✓

// Symptom: Crashes with certain inputs
// Root cause: Assumed data always present

// Bad - assumes email exists
function sendWelcome(user) {
  sendEmail(user.email); // Crashes if email is null ❌
}

// Fixed - validate assumptions
function sendWelcome(user) {
  if (!user?.email) {
    logger.warn('Cannot send welcome email', { userId: user.id });
    return;
  }
  sendEmail(user.email); // ✓
}

// Symptom: Array index errors, missing last item
// Root cause: Loop boundary wrong

// Bad
for (let i = 0; i < array.length - 1; i++) { // Misses last element ❌
  process(array[i]);
}

// Fixed
for (let i = 0; i < array.length; i++) { // ✓
  process(array[i]);
}
// Or better: array.forEach(process);

// Symptom: Date calculations wrong for some users
// Root cause: Not handling timezones

// Bad
const deadline = new Date('2024-01-01'); // Midnight in what timezone? ❌

// Fixed
const deadline = new Date('2024-01-01T00:00:00Z'); // Explicit UTC
// Or use library: dayjs.utc('2024-01-01')

□ Can you reproduce the issue reliably?
□ Do you have the full error message and stack trace?
□ Do you know the exact input that triggers the issue?
□ Have you checked recent changes (git log)?
□ Have you verified your assumptions with logging?
□ Have you isolated the failing component?
□ Do you understand WHY it fails (not just WHERE)?
□ Have you tested your fix against the reproducer?
□ Have you added tests to prevent regression?
□ Have you checked for similar issues elsewhere?

## Issue Description
[What's broken]

## Reproduction Steps
1. [Exact steps]
2. [Expected vs actual]

## Investigation So Far
- [What I've tried]
- [What I've ruled out]
- [Current hypothesis]

## Evidence
- [Logs, errors, screenshots]
- [Minimal code reproducer]

## Environment
- OS, versions, configuration

"Let me try changing this... and this... and this..."
→ You don't know what actually fixed it

Adding print statements everywhere without a plan
→ Noise obscures signal

"Must be a framework bug"
→ 95% of the time, it's your code

Bug: Crashes with large files
Bad fix: Add try/catch to hide error ❌
Good fix: Implement streaming to handle large files ✓

# When process crashes
gdb program core
(gdb) bt          # backtrace
(gdb) info locals # local variables
(gdb) frame 3     # inspect frame

# Capture traffic
tcpdump -i any -w capture.pcap

# Analyze with Wireshark
wireshark capture.pcap

# Or use Charles Proxy, mitmproxy

// Node.js
node --prof app.js
node --prof-process isolate-*.log

// Chrome DevTools
// Performance tab → Record → Analyze flame graph