From a client responding to a prototype with 'lorem ipsum' text on it:
"Why's the site written in Spanish? The partners are definitely going to want to know what that's about!"
Another favorite incident, this during an attempt to explain the screen resolution issue to a client:
Me: "Well, we really don't know what size the user's monitor is going to be..."
Client: "What's the average monitor size?"
Me: "There really isn't one, so..."
At which point the client pulls out a ruler and puts it up against computer screen.