Back when I was a nipper and didn't really understand insurance, I just never cancelled my policy on a car I sold while I was looking for it's replacement. When I had bought my new car about a month later, I called the company and transferred the policy over.
I assume this would be illegal as well ans both myself and the new owner wuold have been insured on my old car at the same, but I honestly didn't know it was wrong so could just have pleaded ignorance...