IIS7 Sessions Getting Crossed / Mixed Up / Copied

I have detected a problem on a web farm:
IIS 7 Sessions of the useres were getting Crossed / Mixed Up / Copied. A user was getting the session of another!!!!

After some research I found that IIS 7 now has the ability to cache dynamic content as well.
It is a new feature in IIS7, this version of IIS introduced some new caching features.
(i) IIS7 automatically caches static content, such as HTML pages, images, and style sheets.
(ii) IIS7 now has the ability to cache dynamic content as well.

In IIS7, disable the caching for .aspx pages in any directory with an asp.net page that depends on the session state. The steps to do this are:
1. Run the Server Management console and navigate to Roles -> Web Server (IIS) -> Internet Information Services.
2. Select the site you wish to modify.
3. Select the folder that contains the .aspx pages you need to turn caching off for.
4. In the Feature View, double-click “Output Caching”.
5. If there is a rule there already for the .aspx extension double click it. Otherwise right click and select “Add…”
6. Enter .aspx for the “File name extension”
7. Check "User-mode caching", "Prevent all caching", check "Kernel-mode caching" and "Prevent all caching"

For more detailed information press here.