Users may not understand the logic applied to their query. Users don't fully understand the subject domain. The subject domain for a commerce site includes the collection of products that comprise the site's catalog, product characteristics and usage assumptions unique to the market, terminology and definitions unique to the market, and schemes for organizing and categorizing the products unique to the market. For example, an avid cyclist who has purchased many bikes and is an expert rider may still lack understanding about how the bicycle industry categorizes bicycles. In the books, music and video retail industry, the subject domain includes how titles are catalogued, indexed, grouped, etc.
Many users employ search as a means of navigation.
This is an indication that the design of a search system must be part of a holistic approach to designing the information architecture of a site. Users are likely to make simple input mistakes. Server search logs show high rates of typographical errors in search terms. Well, it is correctly conjugated, but is the wrong form of the word.