I was on the phone for twenty-four minutes trying to guide a customer through checking her internet connections. Finally she said something odd that led me to the truth: instead of connecting one end of the network cable to her wireless router, and the other end to her laptop... she connected one end to her laptop, and the other end to her desktop PC.
Once she corrected this, suddenly it became easy to see what her problem was: her wifi wasn't connected because she'd accidentally turned it off, using the giant 'wifi on/off' button on her laptop.
"Well", she said philosophically. "Now at least I know what that button is for."
A moment later, while reconnecting the network cable to her desktop PC, she accidentally pushed the network card off the motherboard, so now the desktop PC can't get online anymore.