Late last year, I discussed Verizon's math difficulty. Verizon is far from a unique case, however.

To be fair, before moving on to other businesses, I should mention that Verizon has now fully refunded the customer.

However, that doesn't mean the same problem won't happen again. Already, over at Celebrity Cruises' Staying in Touch page, you can see a similar problem starting. Their default package for internet access aboard their cruise ships is, “$0.65cents a minute.”[sic] Saying $0.65 cents is like telling someone that a package weighs 65 grams in kilograms. It makes absolutely no sense.

Looking at the other rates for context does at least help here. You can get $40 for 75 minutes, which is a decent deal if internet access otherwise is $0.65 per minute (which would come out to $48.75 for 75 minutes). If they actually meant 0.65 cents per minute, then 75 minutes would cost less than 50 cents, and be a far better deal than $40!

Unfortunately, not every math error is so blatant. Imagine you're shopping for 2 DVDs, each of which are $29.99, and you discover the store has a “Buy 1, Get 1 Free” sale. You would expect to pay $29.99 plus tax for the two, correct? One DVD buyer got only 30% off, instead.

Did that last mistake seem too easy to catch? OK, we'll try another one.

In this one, you order a 12-inch pizza for you and your friends. The waitress comes by, explains that your 12-inch pizza was accidentally given to someone else, and apologizes. For the error, she offers the 8-inch pizza she has now, plus a second 8-inch pizza at no charge. Which is the better deal for the same money, the 12-inch pizza or the two 8-inch pizzas? If you need a hint, I'll refer you to the Pizza Theorem.

To find out the answer, I'll refer you to a post by someone who actually faced this decision, entitled, Geometry Saved Me Money.

Sometimes, the error isn't in the cost itself, but in the comparison. Both Quicken Loans and the Gerber Life Grow-Up Plan have fallen prey to this type of error.

Math errors can work both ways. In the next part of this article, I'll discuss businesses who can do the math, but hope that you cannot.