Continuity mistake: Between the time the Professor is put into the trance and waking up, his mouth mouth goes from agape, and head turned to the left, to mouth closed, facing forward.
Continuity mistake: The professor is put under a trance, referred to as "zombiefied." When it first happens, the professor is looking to his left. Later on, he's looking straight ahead.
Answer: Bob Denver answered this when he was on a talk show hosted by Pat Sajak. He said they were going to write into an episode where the castaways found all these clothes, but it got too complicated, so he said, they decided, "who cares, it's only a TV show," and forgot about it. So the writers and actors even asked this, they just never bothered to address it.