I thought certain cliffs or holes led to bottomless areas. Most notably in Rudolph the Red-Nosed Reindeer, I thought that the cliff Yukon Cornelius fell off of was bottomless, and wondered how the abominable snowman could bounce his way up. Ditto with the cliff that Rudoph knocked a log into during the "Fame and Fortune" number.
When I was in the fifth grade I suddenly wondered if my parents had some magic screen where they could watch what I was doing when they weren't around me. I don't know why I thought this (my parents didn't put this thought into my head, for example).
The first time I saw a magic 8-ball in the store, I asked it if I would get a present that day. The 8-ball said yes... And then I didn't get a present that day. Because of that I thought that magic 8-balls were liars (though that didn't stop me from asking for one for Christmas).
When I saw the commercial for the Aladdin video game and it was said that you could get wishes, I thought you really could wish for anything with the game, as opposed to the so-called "wishes" being the ability to choose either an extra life or extra continue. It doesn't help that in the commercial a kid was shown turning somebody into something (I don't recall any disclaimers that you couldn't do that). At the time I didn't know what Frank Oz or Richard Hunt looked like and planned on using the game to wish to know what they looked like.
On a similar note, I thought Wishing Trolls could actually grant wishes. There was a commercial where kids were wishing for things, sort of implying that they'd actually come true, though thinking back on it I don't think they actually showed the kids magically getting their wishes granted. I remember getting a Wishing Troll and wishing it would be Christmas, and not getting that wish.