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.
Forgetting -- then removing -- the daily service charge
You know why the crew on cruise ships tend to all be from other countries? Because cruise ships pay wages that would be illegal in the United States, so they sail under flags from other countries and hire non-US citizens. Nothing sinister about that, but it all works in part because they charge a daily "service charge" that covers your gratuities for maids, room valets, and pretty much everyone else. Be prepared to see a daily charge of anywhere from $15-$30 on your checkout bill, which you ought to pay without objection.
Technically you can have it taken off your bill, if you're the type of person who likes to take a few bucks away from someone making less than minimum wage. Think: If YOU were making $1,000 a month and some dude decided he'd spent too much at the bar and didn't want to tip you, how would you feel?