Experiencing early session timeout (e.g. sudden loss of shopping cart)? If you are on Sitecore 7.2, you should be aware of some known session issues.
The first step you can take is to ensure you are on the latest version (Sitecore 7.2 Update-2 at the time of writing). There is at least 1 known issue with early session termination in Update-1 which has been solved in Update-2.
A related thread on Sitecore SDN can be found here: http://sdn.sitecore.net/SDN5/Forum/ShowPost.aspx?PostID=64360
Another issue has to do with Analytics flagging sessions as robots and terminating prematurely. That issue is described in detail here: http://www.cmsbestpractices.com/sitecore-dialog-temiout-issues/
This issue has not yet been resolved in an official Sitecore update, but the post includes a workaround as well as a hotfix.