The innovation journey of new product development processes often spans weeks or months. Recently, though, hackathons have turned the journey into an ad hoc sprint of only a couple of days using new tools and technologies. Existing research predicts that such conditions will result in a failure to produce new working products, yet hackathons often lead to functioning innovative products. To investigate this puzzle, we closely studied the product development process of 13 comparable projects in assistive technology hackathons. We found that accelerating innovation created temporal ambiguity, as it was unclear how to coordinate the challenging work within such an extremely limited and ad hoc time frame. Multiple projects worked to reduce this ambiguity, importing temporal structures from organizational innovation processes and compressing them to fit the extremely limited and ad hoc time frame. They worked in full coordination to build a new product. They all failed. Only projects that sustained the temporal ambiguity-by working with a minimal basis for coordination and allowing new temporal structures to emerge-were able to produce functioning new products under the intense time pressure. This study contributes to theories on innovation processes, coordination, and temporality.