Stupidity: Considering he had a watch on in nearly every other episode, the Professor did not have his watch in this episode where they had a rescue deadline time (11:00) to be picked up along with the two Russians. Even if he forgot to put his watch on, he definitely would have paid attention to what time it was and got everyone to the location well before 11:00 to guarantee the rescue. Instead he relied on Gilligan to come up with the time which failed (an earlier submitted entry explains that part).
I'd Rather Twitch Than Fight - S3-E10
Stupidity: Like most dilemmas on the show, the controversy could have been resolved by witchcraft without Darrin knowing. Why didn't Samantha just twitch a duplicate tweed coat?