Change in 3rd party requirements.
Regulatory agencies, as well as other groups, tend to change their requirements over time, especially in this age of homology of standards.
What to do about it!
Constantly monitor these 3rd party groups' requirements regarding your products and markets, and provide quick feedback to the PD process (information driven). As an example get on the email list of a reputable compliance test house that supplies timely regulatory agency updates.
Change in social or political climate.
Such changes may require product modification or even targeting a different market.
What to do about it!
Constantly monitor the social and political climates of those markets into which the product is intended to be sold and provide quick feedback of pertinent changes to the PD process.
Change in technology.
What do you do if the technology on which the product IP is based changes during the PD effort? This is a real problem for those high-tech products, which have a long development time relative to the pace of change of the technology.
What to do about it!
Stay up-to-date. Usually, being aware of technology trends and designing the product architecture to accommodate upgraded components will help the product's structure to adapt.