ASP.NET - Problems every time a new cookie is added to existing cookie structure

Sometimes I need to add another cookie to my existing cookie structure but every time I do this the current users get errors when accessing the site.

Having them delete the cookie for my site fixes it.  What is the trick to adding cookies without requiring the users to delete the cookie for my site?

You'd have to post your code that writes the cookie.  No one here has a crystal ball.