Abstract(#br)The concept of gamification has evoked increasing attention in HCI research and practice. Gamification uses game elements in serious, non-game contexts in order to motivate a particular target behavior or attitude change (e.g., sustainable behavior, physical activity, task management). While gamification has been attributed a high potential, a critical question is whether it actually induces the intended effect. The present research explores ``counterproductive effects of gamification'', i.e., cases when a gamification element does not encourage the intended behavior but rather the opposite (e.g., procrastination instead of getting things done). Studying the example of the gamified task manager Habitica, our paper reports insights from two consecutive studies. Study 1, a... qualitative interview study based on interpretative phenomenological analysis (IPA) with one single user revealed seven themes describing distinct counterproductive effects in Habitica, and additional seven themes related to Habitica's reward/punishment system and psychological reactions to counterproductive effects. Study 2 further explored these findings in a quantitative field study with 45 users over a two-week usage period, also studying correlations to user experience, product evaluation, motivation to play Habitica and individual belief in gamification. All participants experienced counterproductive effects to some degree, whereby some effects (e.g., being punished by Habitica in especially productive times, since one does not manage to check off tasks in time) were more prevalent than others (e.g., relabeling tasks as positive habits with no due date to prevent the risk of punishments). The prevalence of counterproductive effects was correlated to the users' perceived inappropriateness of the reward system, and a crucial predictor for motivation change over time. Relations to psychological mechanisms, general implications for gamification design and future research directions are discussed.