I "script" dog-n-pony's (do this; observe that...) so client has a record of everything I showed him and how it all fit together. "Weren't you paying attention??"
Better yet, have the receptionist field his call and reply, "Bowman? Oh, he doesn't work here anymore. Can someone ELSE help you??" and wait for the panic...
Clients aren't really "bad" -- they just don't seem to *think*. I always feel like I know more about their project and market than *they* do! I could understand this if they had lots of irons in the fire. But, often, it's a small startup and this project is EVERYTHING (you'd think they would know it in their sleep given their dependence on it!)