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+# Gotchas & Troubleshooting
+
+Common problems → causes → solutions.
+
+## Permission Errors
+
+### 401 Unauthorized
+
+**Error:** `"401 Unauthorized"`  
+**Cause:** Token missing R2 Data Catalog permissions.  
+**Solution:** Use "Admin Read & Write" token (includes catalog + storage permissions). Test with `catalog.list_namespaces()`.
+
+### 403 Forbidden
+
+**Error:** `"403 Forbidden"` on data files  
+**Cause:** Token lacks storage permissions.  
+**Solution:** Token needs both R2 Data Catalog + R2 Storage Bucket Item permissions.
+
+### Token Rotation Issues
+
+**Error:** New token fails after rotation.  
+**Solution:** Create new token → test in staging → update prod → monitor 24h → revoke old.
+
+## Catalog URI Issues
+
+### 404 Not Found
+
+**Error:** `"404 Catalog not found"`  
+**Cause:** Catalog not enabled or wrong URI.  
+**Solution:** Run `wrangler r2 bucket catalog enable <bucket>`. URI must be HTTPS with `/iceberg/` and case-sensitive bucket name.
+
+### Wrong Warehouse
+
+**Error:** Cannot create/load tables.  
+**Cause:** Warehouse ≠ bucket name.  
+**Solution:** Set `warehouse="bucket-name"` to match bucket exactly.
+
+## Table and Schema Issues
+
+### Table/Namespace Already Exists
+
+**Error:** `"TableAlreadyExistsError"`  
+**Solution:** Use try/except to load existing or check first.
+
+### Namespace Not Found
+
+**Error:** Cannot create table.  
+**Solution:** Create namespace first: `catalog.create_namespace("ns")`
+
+### Schema Evolution Errors
+
+**Error:** `"422 Validation"` on schema update.  
+**Cause:** Incompatible change (required field, type shrink).  
+**Solution:** Only add nullable columns, compatible type widening (int→long, float→double).
+
+## Data and Query Issues
+
+### Empty Scan Results
+
+**Error:** Scan returns no data.  
+**Cause:** Incorrect filter or partition column.  
+**Solution:** Test without filter first: `table.scan().to_pandas()`. Verify partition column names.
+
+### Slow Queries
+
+**Error:** Performance degrades over time.  
+**Cause:** Too many small files.  
+**Solution:** Check file count, compact if >1000 or avg <10MB. See [api.md](api.md#compaction).
+
+### Type Mismatch
+
+**Error:** `"Cannot cast"` on append.  
+**Cause:** PyArrow types don't match Iceberg schema.  
+**Solution:** Cast to int64 (Iceberg default), not int32. Check `table.schema()`.
+
+## Compaction Issues
+
+### Compaction Issues
+
+**Problem:** File count unchanged or compaction takes hours.  
+**Cause:** Target size too large, or table too big for PyIceberg.  
+**Solution:** Only compact if avg <50MB. For >1TB tables, use Spark. Run during low-traffic periods.
+
+## Maintenance Issues
+
+### Snapshot/Orphan Issues
+
+**Problem:** Expiration fails or orphan cleanup deletes active data.  
+**Cause:** Too aggressive retention or wrong order.  
+**Solution:** Always expire snapshots first with `retain_last=10`, then cleanup orphans with 3+ day threshold.
+
+## Concurrency Issues
+
+### Concurrent Write Conflicts
+
+**Problem:** `CommitFailedException` with multiple writers.  
+**Cause:** Optimistic locking - simultaneous commits.  
+**Solution:** Add retry with exponential backoff (see [patterns.md](patterns.md#pattern-6-concurrent-writes-with-retry)).
+
+### Stale Metadata
+
+**Problem:** Old schema/data after external update.  
+**Cause:** Cached metadata.  
+**Solution:** Reload table: `table = catalog.load_table(("ns", "table"))`
+
+## Performance Optimization
+
+### Performance Tips
+
+**Scans:** Use `row_filter` and `selected_fields` to reduce data scanned.  
+**Partitions:** 100-1000 optimal. Avoid high cardinality (millions) or low (<10).  
+**Files:** Keep 100-500MB avg. Compact if <10MB or >10k files.
+
+## Limits
+
+| Resource | Recommended | Impact if Exceeded |
+|----------|-------------|-------------------|
+| Tables/namespace | <10k | Slow list ops |
+| Files/table | <100k | Slow query planning |
+| Partitions/table | 100-1k | Metadata overhead |
+| Snapshots/table | Expire >7d | Metadata bloat |
+
+## Common Error Messages Reference
+
+| Error Message | Likely Cause | Fix |
+|---------------|--------------|-----|
+| `401 Unauthorized` | Missing/invalid token | Check token has catalog+storage permissions |
+| `403 Forbidden` | Token lacks storage permissions | Add R2 Storage Bucket Item permission |
+| `404 Not Found` | Catalog not enabled or wrong URI | Run `wrangler r2 bucket catalog enable` |
+| `409 Conflict` | Table/namespace already exists | Use try/except or load existing |
+| `422 Unprocessable Entity` | Schema validation failed | Check type compatibility, required fields |
+| `CommitFailedException` | Concurrent write conflict | Add retry logic with backoff |
+| `NamespaceAlreadyExistsError` | Namespace exists | Use try/except or load existing |
+| `NoSuchTableError` | Table doesn't exist | Check namespace+table name, create first |
+| `TypeError: Cannot cast` | PyArrow type mismatch | Cast data to match Iceberg schema |
+
+## Debugging Checklist
+
+When things go wrong, check in order:
+
+1. ✅ **Catalog enabled:** `npx wrangler r2 bucket catalog status <bucket>`
+2. ✅ **Token permissions:** Both R2 Data Catalog + R2 Storage in dashboard
+3. ✅ **Connection test:** `catalog.list_namespaces()` succeeds
+4. ✅ **URI format:** HTTPS, includes `/iceberg/`, correct bucket name
+5. ✅ **Warehouse name:** Matches bucket name exactly
+6. ✅ **Namespace exists:** Create before `create_table()`
+7. ✅ **Enable debug logging:** `logging.basicConfig(level=logging.DEBUG)`
+8. ✅ **PyIceberg version:** `pip install --upgrade pyiceberg` (≥0.5.0)
+9. ✅ **File health:** Compact if >1000 files or avg <10MB
+10. ✅ **Snapshot count:** Expire if >100 snapshots
+
+## Enable Debug Logging
+
+```python
+import logging
+logging.basicConfig(level=logging.DEBUG)
+# Now operations show HTTP requests/responses
+```
+
+## Resources
+
+- [Cloudflare Community](https://community.cloudflare.com/c/developers/workers/40)
+- [Cloudflare Discord](https://discord.cloudflare.com) - #r2 channel
+- [PyIceberg GitHub](https://github.com/apache/iceberg-python/issues)
+- [Apache Iceberg Slack](https://iceberg.apache.org/community/)
+
+## Next Steps
+
+- [patterns.md](patterns.md) - Working examples
+- [api.md](api.md) - API reference