What kind of thing?
as described in the book
Consider the case in which the session is used to store the golf course entity instance while the record is being updated in an editor. If you select a course to be edited in one browser tab (or window) and then select a different course to be edited in another browser tab, when you click Save in the first tab, you may inadvertently apply the changes to the wrong course instance. Things get even tricker if you are working with a multi-page wizard, since the leakage of data can be less apparent. It's possible to work around these problems by using the session carefully or putting in extra validation checks to ensure that cross-tab leakage is not occurring, but that is more of a burden on you as a developer.
Could the upper sence happen in seam,if not,what's the reason.
The idea behing Seam is that such a thing doesn't happen. Seam uses
conversationswhich are actively destroyed when the conversation ends, either via a call to a method with the @End annotation or via a timeout. The timeout can be configured in the components.xml file.
After read the subsequence of the book I have a new understanding of seam and thank you for your answer