1) Keep an extra key in your wallet. That way, if you lose your keys inside of a Home Depot, you won’t have to take a cab home.
2) Actually have an extra key for your car, period. That way, if you lose your keys inside of a Home Depot, then go home, you won’t realize you don’t have another key at all. Then you have to call a locksmith to come out and create one for you while you wait in the cold, cold rain.
3) If your house was built before 1975, all projects will take 2-10 times as long. Keep that in mind. Sometimes, just trying to screw something in a wall reveals things previously unknown.
4) When buying a new light (in this case, for our bathroom, over the medicine cabinet) – or anything, really - check to see if it actually fits in the space you plan on putting it. Especially before you install half of it, then realize, hey, why is the top of the medicine cabinet pumping against the bottom of the light fixture?

I know these things seem very dull & boring lessons to learn. But I assure you, you’ll be happier for having known them in advance!