If a failure occurs at the stage of adding a product to the cart, how can this error be characterized?

A failure at the stage of adding a product to the cart can be characterized as:

• Functional error: The main user function - adding a product to the cart - is not performed.
• High/Critical priority error: Affects key business logic, prevents purchase, and requires urgent correction.
• Server-side (Backend) error: Often related to database interaction issues (failed to write, data conflict), API errors (incorrect request/response format, improper handling), or server-side business logic problems.
• User interface (Frontend) / Integration error: May be caused by incorrect request sending from the frontend, server response parsing errors, or frontend-backend interaction issues.
• Validation error: If the failure occurs due to incorrect data (e.g., trying to add a negative quantity, non-existent product), it could be a validation error on either side.

For a more precise characterization, it is necessary to analyze server logs, API requests and responses, as well as interface behavior.