Very sweet: User testing with someone who's never used a computer.

Although the lessons learned are just exaggerated versions of what we all know:
(1) Designers always guess incorrectly at what users actually want to accomplish,
(2) Providing user feedback is important! And users won't always understand memes that industry folk understand.
(3) There's always a dumber user out there than you think. Though being able to guess which users are dumb and which ones are power users, in order to provide them with appropriate discovery tips, is a challenge in itself.