Based on the Smart Cities imaginary, the bottom-up project Stgo2020 created a self-tracking device known as Rastreador Urbano de Bicicletas (or Urban Bicycle Tracker) to record the daily trips of cyclists in Santiago de Chile and use the data gathered to help government officials make better and data-driven decisions on cycling infrastructure planning. In this article, we examine the iterative design of this technology as well as its introduction into the everyday practices of cyclists. We argue that efforts to quantify the ordinary experience of cycling were overwhelmed and interrupted by an ecology of breakdowns, everyday contingencies, forgetfulness, and re-interpretations in the assemblage of devices, data, humans, and bicycles. These breakdowns generated incoherent or absurd bits of information that we call them as ‘‘idiotic data’’ based on recent conceptualizations of the character of the idiot. Significant displacements were provoked by these idiotic data, forcing the engineer behind the device to control and purify the sample by design and algorithms, waning the civic nature of the project at the same time. The case shows how new ways of knowing the urban space by smart devices should be not separated from the emergence of idiotic data, putting into question the versions of citizen participation and smartness at stakes.