When a search is performed, memcache will save the results from that search for a set period of time. After those search results are saved in memcache and the same search term is searched again (either by the same person or even by another person), the same results will show as the previous search, even if the actual results have been changed. This is not user specific. Only keyword and category searches are cached, if you add in another filter (like rating or date), the results will not be cached. This allows the searches to load a lot faster, but can cause, very rarely, altered results than what should really appear.
Full Preview memcache works slightly different than the search memcache, in that the data is not updated automatically forever (almost). It can, however, be updated when a soft-blink occurs (about once a month, or as needed), the user updates the product (changes the description, etc), or when someone rates the product..