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Can you share some real coding bug examples and how you solved them?
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I think one of the best ways to learn debugging is through concrete coding bug examples. Reading about abstract concepts is helpful, but seeing actual bugs and their solutions really sticks with you.
I'm working on a mobile app right now with a weird issue where notifications work on Android but not iOS. It's one of those coding bug examples that seems simple but has layers of complexity.
Could you share some real coding bug examples you've encountered and walk through your debugging process tips for solving them? I'm especially interested in how you approached coding mistake fixes in different programming languages.
Here's a concrete coding bug example from a recent project:
**Bug**: User avatars weren't loading for about 5% of users.
**Symptoms**: The image URLs were correct, the files existed on the server, but some users got 404 errors.
**Debugging process**:
1. Checked server logs - files were being served successfully
2. Checked browser network tab - some requests were getting canceled
3. Noticed pattern: affected users had ad blockers or privacy extensions
4. Discovered issue: Our CDN URL contained tracking" in the domain name (tracking.cdn.example.com)
5. Privacy extensions were blocking requests to domains with "tracking" in the name
**Solution**: Changed CDN domain to "static.cdn.example.com"
**Lesson**: Be careful with domain names and consider how privacy tools might interpret them.
This is a good example of how coding bug examples often involve understanding user behavior and external tools, not just your own code.
**Coding bug example**: Database queries timing out in production but working in development.
**Context**: Python Django application with PostgreSQL.
**Symptoms**: Some API endpoints would occasionally timeout with 504 errors. Database monitoring showed high CPU usage during these times.
**Debugging process tips I used**:
1. First, reproduced locally by increasing data volume
2. Used Django's `connection.queries` to see all SQL queries
3. Found a N+1 query problem in a list view
4. Each item in the list was making separate queries to fetch related data
5. In development with small datasets, this was fast. In production with thousands of items, it timed out.
**Solution**: Used `select_related` and `prefetch_related` to fetch all related data in single queries.
**Coding mistake fixes**:
- Added database query logging to staging environment
- Created performance tests with production-like data volumes
- Implemented query review in code reviews
**Lesson**: Always test with production-scale data, not just development-scale data.
**Real coding bug example**: Java application leaking memory slowly over weeks.
**Symptoms**: Application would gradually use more memory until it crashed with OutOfMemoryError. Restarting fixed it for a few weeks.
**Debugging tools techniques used**:
1. Took heap dumps when memory was high
2. Used Eclipse MAT to analyze heap dumps
3. Found millions of duplicate strings in memory
4. Traced back to a caching implementation that never expired entries
5. The cache was keyed by string, but identical strings from different sources weren't being deduplicated
**Solution**:
1. Implemented LRU eviction policy for the cache
2. Used `String.intern()` for cache keys to deduplicate identical strings
3. Added memory usage monitoring and alerts
**Coding mistake fixes**:
- The original developer assumed the cache would never grow large enough to matter
- No expiration policy was implemented
- No memory monitoring was in place
**Lesson**: Always think about growth limits and expiration policies for caches.
**Coding bug example**: C++ application crashing randomly on customer machines.
**Symptoms**: No consistent reproduction, different stack traces each time.
**Debugging process**:
1. Added extensive logging to customer builds
2. Collected crash dumps from affected machines
3. Found pattern: crashes always involved SSE instructions
4. Discovered issue: We were compiling with SSE4.2 instructions enabled
5. Some customer CPUs only supported up to SSE3
6. The OS was supposed to handle this, but there was a bug in how dynamic linking worked with SSE instructions
**Solution**: Compiled with lower SSE target (SSE3 instead of SSE4.2)
**Coding mistake fixes**:
- Assumed all x86_64 CPUs supported SSE4.2 (they don't)
- Didn't test on older hardware
- Didn't check CPU feature flags at runtime
**Lesson**: Know your minimum system requirements and test on the oldest hardware you claim to support.
**Coding bug example**: Mobile app notifications working on Android but not iOS (your exact issue!)
**Symptoms**: Push notifications delivered fine to Android devices, but iOS devices never received them.
**Debugging process tips**:
1. First, verified APNS certificates were valid and not expired
2. Checked device tokens were being registered correctly
3. Sent test notifications directly via APNS - they worked
4. Traced through our notification service code
5. Found the issue: We were using the same device token cache for both platforms
6. iOS device tokens can change when:
- App is reinstalled
- Device is restored from backup
- User updates iOS
7. Our cache wasn't being invalidated for these cases
**Solution**:
1. Separate caches for iOS and Android tokens
2. Added token validation on app startup
3. Implemented token refresh mechanism
**Lesson**: iOS and Android handle push notifications very differently. Don't assume what works for one works for the other.
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