It seems that .Net’s garbage collection retains a reference to the SqlParameter used in the first SqlCommand. This reference seems to survive the closing and even disposing of the connection in use. As long as the SqlCommand object is in use, .Net seems to remember that it has already been used. To bypass this, simply call the sqlCommand.Parameters.Clear() method. You’ll need to readd the SqlParameter or SqlParameter[] to the SqlCommand and it’ll work.