You're Doing it Wrong
Prepare to learn that you've been doing something wrong your entire life. In an extensive thread at Ask Metafilter, commenters share what they did wrong for years. The list is full of spelling and pronunciation problems, but many go much deeper. Here are some of my favorites:
I just found out last month that you are not supposed to grip a bowling ball with your thumb, index finger, and middle finger; it should be your thumb, middle finger, and ring finger. For years I wondered why my ball tended to curve into the gutter rather than go straight down the lane like everyone else's...
posted by castlebravo
One of my friends looked like he had a major epiphany when another friend taught him the trick of making a letter L on his left hand to work out left from right. His expression was priceless.
posted by divabat
One day I noticed that one of my favorite t-shirts had a tiny little hole near my belly button. Not a big one. Just a tiny little hole. I didn't think much of it. But then I noticed it on another t-shirt. Some shirts had more holes than others. None of my nicer shirts had holes, but all of my more everyday-casual-wear-all-the-time shirts had holes. I decided that it must be because they were so cheap (and they were very cheap t-shirts). So I upgraded. But it wasn't long until I noticed a hole on the new shirt. For years the reason behind these tiny holes eluded me and so I assumed it must be my abs.
So one day I'm playing softball with my friends. It's a warm evening and I reach into the cooler of beer and pull out an ice cold bottle of beer. The kind with a twist off cap. I'm joking with my friends and I look down and realize that everytime I'm opening a bottle, I'm grabbing my t-shirt and twisting the cap off with the shirt. And all of the holes are right where I grab the bottle and twisted the cap off.
So it took me a while to learn not to open a bottle with my shirt in hand.
posted by phelixshu
[Responding to this comment:] Rotating your tires is no joke. Do it if you don't want to buy a whole new set every year even though only 2 are worn.
I was 24 (so roughly 8 years of car ownership) before I finally figured out that this means "switch the positions of all 4 tires on your car for equally distributed wear" and not "jack the car up and manually spin the tires for no apparent reason".
posted by elizardbits
And there are hundreds more. Feel free to share your own in the comments here or on Metafilter!