Unplanned category purchase incidence is an important source of retailer volume and profits. We analyze this phenomenon in detail with a multi-level Poisson model calibrated on data from 434 households making over 18,000 purchases in 58 categories across 3,000 trips to 21 stores. We find that unplanned category purchase incidence is not proportional to the number of categories bought, and not a single shopping trip is completely unplanned. The majority of variation is across shoppers. Specifically, it is explained in part by demographic variables traditionally measured by marketers, but more by other "traits" that reflect long-run shopping habits such as level of planning and information gathering styles. Short term shopping goals (e.g., major trip, forgotten needs, etc.) also play an important role. We replicate classic results of time available (more unplanned purchasing) and high store knowledge (less unplanned purchasing); however we find that the overall occurrence of unplanned purchasing is significantly lower than that commonly reported.