This study is based on the Activity Theory and observed a Brazilian bank, describing the contradictions raised by netnography in an activity system - one of the bank’s employee online communication channels, called the Collaborative Forum. In this channel, employees exchange information, problems, and suggestions related to their activity. A specific topic for this investigation was created in the channel, and based on excerpts from the speeches, it was possible to identify tensions indicating contradictions, categorize them, and formulate contradiction hypotheses. This information supported a set of mirror data that provided initial empirical material for structuring this case. Netnography stands out as an innovative method in studies on formative intervention. Together with the theoretical-methodological foundations of the activity theory, netnography proved to be a potential tool for producing mirror data, providing opportunities for a preliminary survey of hypotheses that could help the interventionist researcher better understand an activity’s contradictions as a whole.