Functional Bug

Functional bugs are the most straightforward category of defects. The feature is supposed to do X, but it does Y instead, or it does nothing at all. Examples include a search that returns wrong results, a filter that does not actually filter, a sort that does not sort correctly, or a form that submits but does not save the data.

In vibecoded applications, functional bugs often appear in the gap between what was described in the prompt and what the AI actually implemented. The developer might ask for a form that validates email addresses, and the AI might implement a validation that checks for an @ symbol but accepts clearly invalid formats. The feature technically does something, but it does not do it correctly.

Functional bugs are the primary focus of QA testing because they directly affect whether users can accomplish their goals. A well-structured test session systematically verifies each feature against its intended behavior, catching functional bugs before they frustrate real users.