What common mistake do developers make with Api route middleware, why does the mistake cause problems, and what is the safer approach?

What common mistake do developers make with Api route middleware, why does the mistake cause problems, and what is the safer approach? One recurring problem with API route middleware is applying it without understanding the boundary it protects. Returning HTML error pages from a JSON API without considering content negotiation and client expectations. In addition, API routes can share middleware such as authentication, throttling and request normalization. Safer engineering approach: keep the responsibility in the correct layer, use the framework's supported API, validate and authorize inputs, avoid hidden side effects, and verify the behavior with tests.