This happens to me often, especially when I'm visiting relatives' houses: My computer knows the Wi-Fi password from when I last connected to the network -- two years ago -- but I've completely forgotten it, though my mother is about 65 percent sure she wrote it down on a Post-it note seven months ago.
A forgotten Wi-Fi password isn't a problem for me and my omniscient laptop, but it is a problem for, say, my brother, who needs the password if he wants to jump on the network with his Japanese iPhone. My mom is hunting for the Post-it, but prospects are lookin' bleak.