When I was a kid, I read an article of Martin Gardner (in a book of collected columns originally published in the 1960s) observing that many textbooks "prove" that air is 20% oxygen by burning a candle in a plate of water, with a jar inverted on top. When the flame goes out, the water rises into the jar, to a distance of about 20% (of course, nobody measures this exactly). That, apparently, "proves" that the used-up oxygen constituted 20% of the air. Of course, this had been in my textbooks too (and even a class demonstration), and nobody had thought to question it.
A ten-year-old tends to be a bit uncritical of such "arguments". But surely anyone older -- such as the teachers teaching this -- would know that (a) The candle would't use up all the oxygen; (b) The oxygen is being used up in making carbon dioxide and water vapour, which are both gases (and water molecules, unlike oxygen molecules, have a single atom of oxygen), so if anything the volume of gas should have increased inside the jar; (c) the heat should have caused the air to expand; (d) the heat should have caused some water to evaporate from the plate, adding to the volume of air in the jar.
The only reason the water rises into the jar is (c) above. When you invert the jar on top of the candle, the air around the candle is already hot. When the candle goes out, the air cools, contracts and sucks in the water.
Another question was why do stars twinkle? One book attributed it to the stellar equivalent of solar flares, which is nonsense. Thankfully that was the exception: other books said, correctly, that atmospheric turbulence "bends" the starlight. But then why don't planets twinkle? Not one textbook that I remember explained the reason. (The reason is that stars are so far off they are effectively point sources of light, while planets are more disc-like -- one can see the discs with a quite modest telescope -- and the amount of atmospheric bending is small compared to the diameter of the disc.)
Today I came across another such bogus explanation posted by a commenter on Space Bar's blog in response to her (rhetorical?) question "Why do smells sit so heavily in the summer?" The explanation was that air moves faster, and carries smells more quickly. Plausible at first thought, but the math doesn't work out (as I show in my reply there). But I expect there are several textbooks out there that trot out some such explanation all the same.
How many such bogus "scientific" answers were there, and are still there, in school textbooks that I have forgotten about (or continue to believe credulously)? Richard Feynman reports regularly blowing up like a volcano when asked to review some California state school science books. I suspect my own temper may be hard to keep when my son starts learning science in school.
But no textbook, however atrocious, could compare with the the unauthorised "guide books" / "sample questions and answers" that were, and are, a staple of kids preparing for exams. The following, I remember, was a source of hilarity for my brother. "Question: Though India's north-south dimensions are about the same as its east-west dimensions, the difference in longitudes between east and west is much greater than the difference in latitudes between north and south. Why is that? Model answer: Because Bangladesh comes in the middle."