Quote Originally Posted by RodG View Post
The most likely effect of this will be that they'll suddenly be greeted with a 'session expired' type message and will be required to log in again.
If the customers aren't logged it, they'll suffer a 'hiccup' and basically lose anything already placed in their cart and will need to re-add them.

Then there are the unpredictable effects of what could happen if something was being done at exactly the 'wrong' time (such as midway through a paypal payment).
Since the setting only affects the creation of subsequent cookies (not the enforcement of any matching, since that's all up to the browser), it should have no effect on existing active shoppers. That is, it won't break active working sessions.