One of the ones that I remember vividly is being told as a teenager that "crushes" on people were a normal part of adolescence (ok so far), but also something you grow out of when you start having "proper", "adult" relationships. Now, I dunno about anyone else, but I consider this one of the biggest lies I was ever told. It bothers me far worse than Santa Claus or anything like that, because it did me actual harm. According to the lie, as soon as I became an adult and started having proper reciprocated relationships, I'd stop having random feelings for people I didn't necessary know very well. But this never happened - my attraction circuits never shut off. So I spent 3 years trying to do monogamy and thinking that I was broken or not trying hard enough because I was still attracted to people apart from the person who was supposed to be my partner.
The reason I'm polyamorous now is because I realised that trying to do monogamy breaks me - I can either lie to myself about how I'm feeling (and make myself miserable), or lie to my partner about what I'm feeling (and make them miserable). But the crush thing isn't limited to people who seem to be wired for non-monogamy - even monogamous people in happy, committed relationships still have occasional weird strong feelings for other people who might be unobtainable and with whom they know full well a relationship might never work. So what's going on there? And crushes can go on for years. I've had a local friend who I've had a crush on in one sense or another for going on 9 years - but we've never tried to have a relationship because it so clearly wouldn't work. Common sense doesn't stop the occasional wistful feeling, though.
So why do parents or teachers insist on lying to teenagers? Is it all tied up with the myth of The One True Love? (everyone has a soulmate who is their Other Half, and they need to meet that person to become Whole, and once met they should never be parted etc etc yadda yadda puke). Is it desire to help the teenager get through a difficult couple of years, or designed to harm the young adult who finds themself taking a different path? Or wishful thinking on the part of a parent who has been happily married for 20 years? (yeah right, how many of us had them?).