The best/worst has to be:
Cats can either lap up liquids when they drink OR they can curl their tongue and use it as a straw if they are thirsty and want to drink quickly...
I am 20, I told this "fact" to some children I was looking after a few months ago, their parents wet themselves laughing when they heard! An illusion only recently broken!
But then again I have also told some pretty idiotic things to my sisters:
Maggots taste of chocolate (we were fishing using maggots at the time... she tried them... I got in quite a lot of trouble for that one!)
Many years ago, I had a mare that only had a wisp for a forelock. I sold her to my best friend.
Fast forward to 6 years ago, she bought her mare that she has now and made a right song and dance when another livery offered her mare a bit of cabbage. Apparently, when she bought the mare, I told her she had a teensy forelock because she'd been given cabbage to eat! She believed me, and been vigilant against it, for all those years, right up to the age of 42!
(in my defence, I don't remember saying it, but it is the sort of daft thing I would have made up!)