Firms use ideation contests to generate ideas from consumers. This type of collaboration provides access to new knowledge and reveals latent consumer needs. But it also is risky, as firms give up control to an unknown crowd. Some contestants use ideation contests to post content that is unintended and unwanted by contest hosts, a behavior that represents deviant co‐creation. Drawing on literature from sociology and consumer research, deviance is defined as a relative, norm‐violating behavior that has the potential to activate others. We report the results from a netnography study to define the phenomenon of deviant co‐creation in ideation contests. Based on these findings, we provide a theoretical foundation for deviant co‐creation and conceptualize and empirically illustrate various patterns of deviant content, ranging from destructive to constructive. The study reveals that deviant content in ideation contests includes illegitimate as well as legitimate content. Legitimate content includes five themes: humorous, provocative, unique, violation from technical, and social norms. Deviant content usually bewilders evaluators and draws their attention to the content. Destructive deviant content may trigger visible and malicious protests or result in mocking and ridicule on the contest platform and other social media, thereby exposing the contest host to reputational risks. Constructive deviant content can lead to positive discussions in comment sections and other social media outlets, as well as foster further development of an initial idea, thereby contributing to the firm's innovation potential. This article provides managers a deeper understanding of deviant content raising awareness for the dark side risks as well as indicating how to leverage it to achieve constructive co‐creation.