What common mistake do developers make with Testing scopes, why does the mistake cause problems, and what is the safer approach?

What common mistake do developers make with Testing scopes, why does the mistake cause problems, and what is the safer approach? One recurring problem with testing scopes is applying it without understanding the boundary it protects. Making a global scope responsible for business rules that should only apply to one reporting screen or workflow. In addition, Scopes can be tested against realistic data to ensure their conditions match business expectations. 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.