July 27, 2013  -- Have you ever checked your monthly credit card bill, only to notice a charge you didn't expect to see? If you have, you're not alone. According to a new study from software company BillGuard and research firm Aite Group, there are roughly 233 million hidden credit card charges and fees each year, costing cardholders an estimated $14.3 billion in 2012 alone. Thirty-five percent of the nearly 5,000 BillGuard users in the study were affected by these "grey charges," as the company called them, averaging in at $61 per transaction. (Billguard's personal finance security service uses crowd sourcing technology to identify grey charges to participating consumers.)

There are several ways companies can slide a grey charge past you, and unfortunately, they're all technically legal. The most common charge is "free-to-paid," racking up $6.1 billion over 115 million transactions. This type of charge occurs when a seller automatically charges a consumer a fee for their service after a trial period, oftentimes without notifying the consumer that they had to cancel their account to avoid payment.

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Other types of grey charges include misleading advertising, with unsubstantiated claims, false testimonials, or incomplete descriptions, service fees, unintended subscriptions, and more. These charges are fairly consistent across the board, with little variance from company to company. More often than not, miscellaneous fees and costs are hidden somewhere in the jargon of a company's terms of service agreement, and most consumers are unaware of what they're signing off on.

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Though charges fall on consumers, these practices harm credit card companies, too. The study estimates that banks generate about $214 million in fees annually, but with 1 in every 3 of the 190 million debit and credit cardholders in the United States impacted, companies see a sizable number of customer service complaints.

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Card issuers incur an average of $562 million in customer service-related costs each year. This is more than double the amount of revenue generated for card issuers from these fees, even before accounting for charges that are disputed and eventually reversed. And an increase in consumer awareness surrounding these charges could lead to more disputes and higher costs for credit card companies.

While credit card companies oftentimes advertise their fraud protection services, grey charges may not even register as irregular in monthly statements. This is why it's so important for you to regularly examine each credit card bill, to be sure you're not paying for something you haven't bargained for. Keep your receipts, cross-reference them against your statements, and be sure to clarify your contract with any subscription services you may have.

This work is the opinion of the columnist and in no way reflects the opinion of ABC News.