If you find something that does not belong or got double charged, take your receipt to the pursers desk and they will fix it for you. It does happen, but not as often as the posts you are reading make it seem. Remember, people only tend to post a negative experience. Compare the number of bad S&S posts you see with the 10s of thousands of passengers that sail on Carnival in a given week... that will put it in perspective.
Thank goodness that has not happened here. It has only happened to me a few times, and I think LHP said it only happened to her once or twice. And she has "way" more cruises under her belt than me. The point is to check the account for any errors. As DrL noted it is easier to address this before the last day should it occur.