Review the following code for:

1. **Bugs & Edge Cases** - Logic errors, null checks, boundary conditions
2. **Performance** - Inefficiencies, unnecessary operations
3. **Security** - Vulnerabilities, input validation, data exposure
4. **Readability** - Naming, structure, documentation
5. **Best Practices** - Language idioms, design patterns

Code:
```[LANGUAGE]
[PASTE CODE HERE]
```

For each issue found:
- Explain the problem
- Show the problematic code
- Provide the fixed version
- Rate severity (Critical/Medium/Low)