I have had that happen to many of my clients in the past. They get the renewal notices from the registrar and never do anything with it. A few months later after the domain has expired and been bought by somebody else they come complaining to us.
It always seems that these domains are the ones with great rankings and traffic as well.
One of my clients (outdoor furniture manufacturer) let THE PRIME keyword dotcom go. He called me in a panic. I told him the solution was very simple, give me the authority to buy it back, sell his house, then send me the money so I could start bargaining.