There used to be an online version of MegaHAL whose brain was wiped clean every 24 hours. One day when I got there just after the brainwipe, I fed him advice from a horoscope site for every sign from Aries to Pisces. When I checked the daily log later, I discovered that someone else--who'd come in just after I had--had a frighteningly lucid conversation where HAL gave him/her some really good advice. It's the only time I've ever seen HAL do a convincing job of impersonating a human being.