Cart contents mixed together

Hi, we've recently gone live with CS cart and one of our customers has complained that she lost some items in her cart when clicking update, and that some other products were added in too.

I seem to remember a setting for the cart being tracked by i.p. address or cookie - does anyone remember where this setting is?

Im thinking our store may track cart sessions by i.p. address and that contents are being thrown in together by having one than one computer using the same internet connection.