Expecting to use Wi-Fi
You will never, ever complain about having to pay $19.95 for in-room internet at a fancy hotel again. On many cruises, it costs upward of $25… per hour. And it moves at roughly the speed of 1997 dial-up, so in that hour you'll be lucky if you can return three emails and see half a boob. Some newer ships have faster -- and cheaper -- connections, so ask what the Wi-Fi situation is when you book. Or, do something completely crazy and disconnect for a few days. We guarantee after day two you'll wonder why you don't do it every vacation.
Bringing booze on board -- without planning
Like we said, cruises make a ton of cash on your onboard liquor purchases, so your bags will be X-rayed and searched for boozy carry-ons. You are allowed to bring a limited amount of wine on board, but the corkage fee at the restaurants is about on par with what a bottle of wine costs there. So pick your poison.
The best method is to buy bottles of liquor beforehand and transfer them into clear plastic pouches (also called mini-flasks), then spend the weekend mixing it with the free lemonade served in the dining room. Is it classy? No. Will it keep you entertained during an at-sea day without running up a $250 bar tab? Probably.