Poor or Incomplete Documentation
One of the most frequent complaints from API consumers is a lack of clear documentation. Without detailed guides, examples, and error descriptions, developers are left guessing. To avoid this, provide comprehensive and up-to-date documentation using tools like Swagger or Postman. Make sure to include usage examples, authentication methods, rate limits, and common error responses.

Ignoring Error Handling and Status Codes
Returning vague or incorrect status codes (e.g., always returning 200 OK) makes debugging extremely difficult. Proper error handling with meaningful HTTP status codes (like 400 for bad requests or 401 for unauthorized access) helps clients respond appropriately and improves the developer experience.

Not Versioning Your API
APIs evolve, and breaking changes are sometimes necessary. Without versioning, you risk breaking existing integrations. Implement versioning in your URL structure (e.g., /api/v1/) or through headers to allow smooth transitions and backward compatibility as your API grows.

Overlooking Security Practices
Many APIs are vulnerable to threats like injection attacks, data leaks, or unauthorized access. To avoid this, always validate inputs, use HTTPS, implement authentication (OAuth 2.0, API keys, etc.), and monitor for unusual activity. Security should be baked into every stage of development—not added later.

Designing Inconsistently
Inconsistent naming conventions, data structures, or response formats confuse developers and increase integration errors. Stick to a well-defined design standard (like RESTful conventions), and ensure consistency across all endpoints for a smoother experience.

Conclusion
A well-designed API is not only functional—it’s secure, predictable, and developer-friendly. By avoiding these five common mistakes, you’ll build APIs that are easier to use, easier to scale, and more likely to be adopted by developers. Taking the time to get it right can save countless hours of troubleshooting down the road.