Factual error: The portable radio is an AM band only, therefore there is no need for the external telescoping FM antenna.
The Friendly Physician - S2-E29
Factual error: When the castaways are coming back to the island at the end, you can see some of the Los Angeles buildings in the background in the high camera angle.
Topsy-Turvy - S3-E10
Factual error: When looking at the machetes the headhunters had, you can see the molded plastic handles on them. The natives wouldn't have access to such things.
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.