This paper is concerned with how forms of publics come into being in situations of technological innovation and change. The paper attends to sudden social formations which arise as responses to what is perceived of as threatening socio-technical inventions into the routine of everyday social life. We revisit the Dewey-Lippmann debate, and we look at spontaneous citizens’ reactions towards emerging technologies such as Google Street View as well as we discuss some institutionally organized events of public engagement with science. As we explore the formation of publics inside and outside various institutional contexts, we suggest a concept of mobilizing unexpected agencies that we call the public spectre. Crowds emerge spontaneously, and assume the figure of a ‘public spectre’ that resides in the unforeseen. When repeated as collective events, crowds stabilize and assume the figure of publics. The notion of the public as a spectre draws attention to the plurality of forms in terms of which publics emerge and take form.