Hi everyone! 👋
I'm a .NET developer with 5 years of experience, and during my career, I've worked on many projects involving REST APIs—some small, some large, and all with their own learning curves.
Today, I want to share my practical experience by discussing the Top 15 Mistakes Developers Make When Creating APIs.
If you’re a beginner, or even if you have some experience, I hope this post helps you avoid common pitfalls and build better APIs!

1. Not Following API Design Standards

Mistake:
Ignoring standard conventions (like RESTful guidelines) and inventing your own style.

Why it matters:
Standards exist for a reason—they make your API predictable, easier to use, and easier to integrate with other systems. If you don’t follow them, you’ll confuse consumers of your API and possibly yourself later.

.NET Perspective:
Use RESTful routing in ASP.NET Core ([HttpGet], [HttpPost], etc.), and pay attention to resource-based endpoints.

Example:

Tip:
Read the Microsoft REST API Guidelines and the ASP.NET Core Web API documentation.

2. Lack of Proper API Documentation

Mistake:
Not providing documentation, or only writing minimal docs.

Why it matters:
No one can use your API (including you, 6 months from now) if they don’t know how it works. Good docs save time for everyone.

.NET Perspective:
Use Swagger/OpenAPI (Swashbuckle) with ASP.NET Core. With just a few lines you get interactive docs online.

Example:

// Startup.cs
services.AddSwaggerGen();
app.UseSwagger();
app.UseSwaggerUI();

• Include sample requests and responses.
• Document authentication, error codes, and edge cases.

3. Ignoring Versioning

Mistake:
Publishing your API without a version, then breaking clients when you make changes.

Why it matters:
APIs evolve. Without versioning, even a small change can break all existing consumers.

.NET Perspective:
Prefix your routes with a version number (e.g., /api/v1/products). Use API versioning libraries like AspNetApiVersioning.

Example:

[ApiVersion("1.0")]
[Route("api/v{version:apiVersion}/products")]

4. Not Validating Input Data

Mistake:
Trusting that clients will always send good data.

Why it matters:
Invalid input leads to data corruption, exceptions, or security issues.

.NET Perspective:
Use Data Annotations ([Required], [StringLength], etc.) and model validation (ModelState.IsValid).

Example:

public class ProductDto
{
    [Required]
    public string Name { get; set; }
    [Range(0.01, 10000)]
    public decimal Price { get; set; }
}

5. Weak Authentication and Authorization

Mistake:
Leaving your API open or using insecure authentication.

Why it matters:
Unprotected APIs are easy targets for abuse and data theft.

.NET Perspective:
Use ASP.NET Core Identity, JWT bearer authentication, or OAuth2. Secure all endpoints that need protection.

Example:

[Authorize]
[HttpGet("orders")]
public IActionResult GetOrders() { ... }

Tip:
Always use HTTPS. Never accept credentials over HTTP.

6. Inconsistent Naming Conventions

Mistake:
Mixing naming styles or using unclear names for endpoints and fields.

Why it matters:
Consistency helps everyone understand and use your API without confusion.

.NET Perspective:
Stick to camelCase or PascalCase (but be aware JSON often uses camelCase). Use plural nouns for collections (/products not /productList).

Example:

7. Poor Error Handling

Mistake:
Returning unhelpful error messages, or leaking sensitive stack traces.

Why it matters:
Good error handling helps clients debug issues. Leaking technical details can expose vulnerabilities.

.NET Perspective:
Use proper HTTP status codes and return structured error responses.

Example:

{
  "error": "PRODUCT_NOT_FOUND",
  "message": "Product with id 123 does not exist."
}

• Use middleware for global error handling.
• Don’t expose exception details in production.

8. Incorrect Use of HTTP Status Codes

Mistake:
Returning 200 OK for every response, even errors.

Why it matters:
Status codes communicate what happened to the client. Misusing them confuses consumers and breaks integrations.

.NET Perspective:
Return 201 Created after successful POSTs, 204 No Content for deletes, 400 Bad Request for validation failures, 404 Not Found for missing resources.

Example Table:

9. No Rate Limiting

Mistake:
Allowing clients to make unlimited requests.

Why it matters:
Without rate limiting, your API can be brought down by accidental overuse or malicious attacks.

.NET Perspective:
Use middleware (like AspNetCoreRateLimit) to enforce limits.

Example:

• Limit each IP to 100 requests per minute.
• Return 429 Too Many Requests if limits are exceeded.

10. Not Using HTTPS

Mistake:
Letting clients access your API over HTTP.

Why it matters:
HTTP traffic can be intercepted. Sensitive data (like tokens or personal info) can be stolen.

.NET Perspective:
Enforce HTTPS redirection in your app.

app.UseHttpsRedirection();

11. Overcomplicated or Under-designed Endpoints

Mistake:
Trying to do too much in one endpoint or having endpoints that are too granular.

Why it matters:
Complex endpoints are hard to use and maintain. Under-designed ones force clients to make many calls.

.NET Perspective:
Design endpoints around resources and actions, not implementation details. Provide filtering, sorting, and pagination where needed.

Example:

| Good: /api/products?category=books&page=2 | Bad: /api/getProductsByCategoryAndPage |

12. Not Implementing Pagination

Mistake:
Returning all results in one response for list endpoints.

Why it matters:
Large responses slow down your app and can cause timeouts.

.NET Perspective:
Implement pagination using query parameters (e.g., ?page=1&pageSize=20). Return metadata about total records.

Example:

{
  "page": 1,
  "pageSize": 20,
  "totalItems": 200,
  "products": [ ... ]
}

13. Hardcoding Secrets in Code

Mistake:
Storing API keys, connection strings, or passwords directly in your code.

Why it matters:
If your code is ever shared, your secrets can be leaked.

.NET Perspective:
Use appsettings.json, environment variables, or Azure Key Vault. Never commit secrets to Git.

Example:

// appsettings.json
"Jwt": {
  "Key": "UseEnvVarOrKeyVaultHere"
}

14. Breaking Backward Compatibility

Mistake:
Changing or removing endpoints/fields without warning.

Why it matters:
Existing clients will break if you make incompatible changes.

.NET Perspective:
Only make breaking changes in new API versions. Deprecate old endpoints with clear warnings.

Tip:
Document deprecations and provide migration guides.

15. Ignoring Security Best Practices

Mistake:
Not validating input, not checking permissions, exposing too much data, or not updating dependencies.

Why it matters:
APIs are common targets for attacks. Security breaches can be catastrophic.

.NET Perspective:

• Sanitize and validate all input.
• Use [Authorize] everywhere sensitive.
• Limit what data is returned (no sensitive fields in DTOs).
• Regularly update NuGet packages and monitor for vulnerabilities.

Security Checklist:

• All endpoints require authentication unless explicitly public.
• Sensitive data never sent in URLs.
• Input is validated and sanitized.
• Secrets are never hardcoded or logged.

Real-World Examples from My Experience

A. The Forgotten Version

Early in my career, I published an API without versioning. Months later, business requirements changed, and I had to update several endpoints. Suddenly, all clients broke. Fixing this took hours of apologetic emails and frantic patching. Lesson learned: Always version your API from day one!

B. The "It Works on My Machine" Error

I once returned raw exception messages to the client for easier debugging. Then one day, a production crash exposed a database connection string in the error response. Thankfully, it was caught quickly, but it could have been a disaster. Now, I always use friendly, non-sensitive error messages.

C. The Overload Outage

Our API didn’t have rate limiting. One day, a misconfigured client started spamming thousands of requests per minute. The server didn't crash, but it slowed to a crawl for everyone. Adding rate limits was a quick fix, but I wish we’d started with it from the beginning.

Conclusion

APIs are everywhere—you’ll probably build, use, or maintain several in your career.
Avoiding these 15 mistakes will make your APIs more robust, secure, and enjoyable for others to use.

Quick Recap:

1. Not following API standards
2. Poor documentation
3. Ignoring versioning
4. Not validating input
5. Weak authentication/authorization
6. Inconsistent naming
7. Poor error handling
8. Incorrect use of status codes
9. No rate limiting
10. Not using HTTPS
11. Bad endpoint design
12. No pagination
13. Hardcoded secrets
14. Breaking compatibility
15. Ignoring security

My advice:
Start with good habits. Use the tools ASP.NET Core gives you (Swagger, versioning, authentication, validation).
Read docs, follow good examples, and always put yourself in the shoes of someone who has never seen your API before.

Further Reading

• Microsoft REST API Guidelines
• ASP.NET Core Web API documentation
• OWASP Top 10 for APIs
• Swagger/OpenAPI for .NET