A common mistake inventors make is to assume that because an invention is patentable, it is also necessarily non-infringing.  This is not the case, however.

Often an improvement in the art, while itself patentable, falls within the scope of previous patents.  As an example, consider the case of an inventor who invents the combination of elements A+B+C+D in view of the state of the art represented by A+B+C.  Assuming the combination of D with the state of the art is not obvious, the new combination is patentable.  However, another party may have an issued patent containing claims to the combination A+B+C, rendering the new improvement both patentable and infringing.

What is moral of this story?

Consider having a clearance search performed rather than merely a patentability search.  Although a clearances search may cost more upfront, the extra investment in ensuring product clearance can save significant costs (e.g., for infringement defense and product re-design) down the road.