The majority of IE cookie issues are revolved around the number of cookies stored per domain. Prior to the August Cumulative Update for Internet Explorer, the IE cookie jar stored a maximum of 20 cookies per domain. If more than 20 cookies were sent by the server, older cookies were automatically dropped by the browser. This created a situation where a user could experience lost site settings, an empty web shopping basket or other issues.
As part of the IE Update, the cookies per domain limit has been increased from 20 to 50. The change was made to simplify the development and hosting of web applications. There's a much more detailed post over at the IE Blog regarding this update.
- KB 306070: Number and Size Limits of an IE Cookie
- KB 820536: Document.Cookie Property Returns an Empty String
Source:→ Performance Team Blog