There are at least few that I purchased that I cancelled few years later since I dropped the ideas and I couldn't sell the names either. Getting $5 for them and wasting time pushing them was not interesting, so I just cancelled them. I still have about 80 names I presume, I don't even know to be honest, and just few are used as active sites, most are used to protect key domains in different variations.

However, I never regret purchasing a domain at the time since there is usually a plan for it. Its a small price to pay to know the name will be secure for a while.