Overpacking/Exceeding the Airline's Weight Limit
First of all, if you can't carry it yourself, don't pack it. Whatever you manage to stuff in your suitcase is going to be your responsibility for the duration of your trip and there may or may not always be a place to store it (and nobody wants to smell your dirty shoes as you try and navigate through tight spaces with the contents of a small Gap store strapped to your back).
This is less an issue if you're staying at the Disney World resort for the weekend than say, taking a river cruise on the Mekong, but it's still important to know what you can carry and how much the airline will charge you to bring it along.
BEST BET: Pack the essentials and grab anything you may have forgotten on the fly. Worst case scenario you end up with a souvenir sweatshirt at the end of your trip.PRO TIP: Don't exceed the max weight limit allowed by your airline for checked baggage. That can result in $100 extra EACH WAY and the truth is you don't NEED a liter of shampoo for your six-day cruise.
Buying Foreign Currency at the Airport
By bus, train, or taxi, you're going to need at least a ride's worth of cash to get out of the airport in many cities (unless you've got a really low cut shirt and a dwindling cache of moral sensibility, in which case bargain away). In countries where the local currency isn't worth as much as say, the US dollar or the British pound, cabbies will take your home currency, but they're not doing it out of the kindness of their hearts (read: it comes with a fee). Just know that, despite whatever bargain rate or free exchange service the kiosks at the airport are advertising, you're not getting the best deal.
BEST BET: Either order currency from your bank before you go or hit up a local bank once you're closer to your final destination.PRO TIP: Don't order too much cash to begin with because if you don't spend it all, your bank is going to take a nice little cut when you get back and try to exchange it for your home currency. And in case it's been awhile since you've traveled and you're thinking they're still a thing, no, traveler's checks are not the answer.
Leaving Your Power Converter at Home
This is just the worst, period. Imagine arriving at an international airport with a dead phone or laptop and having no idea where you're going. True, there are many airports that now have stations to plug in via USB connection, but once you get to your lodging, you're going to want one of these without the hassle of finding a store that sells them for any amount less than your first-born child.
BEST BET: Invest in a universal charger and keep it with your travel gear.PRO TIP: Get one with USB connectors so you can charge more than one device at a time.
Not Checking the Weather of the Place You're Visiting
Guess what: once a year it rains in Southern California. Being caught without the proper apparel on that lone day in April is forgivable, but think twice about traveling to other parts of the world without checking what season you're in for (as it may or may not match with your home location). A duffel's worth of tank tops isn't gonna do squat for you during monsoon season.
BEST BET: A collapsible poncho is worth shoving one fewer pair of socks in your bag.PRO TIP: Always have at least the base layers of the most extreme weather you could encounter (hot and cold).