As the holiday shopping season kicks into high gear, consumers should be mindful of the tricks retailers use to get them to spend more money.

Using a web crawler on over 11,000 retailer websites worldwide, a team of researchers from Princeton University and the University of Chicago discovered so-called "dark patterns" on more than one in 10 sites, which can trick people into signing up for recurring subscriptions and buying things they don't want. In a new report, released this November, the researchers also reported that "many" of these "dark patterns might be "potentially unlawful."

That's not necessarily breaking news — the report, called "Dark Patterns at Scale: Findings from a Crawl of 11K Shopping Websites," notes that market manipulation techniques have long been used by retailers to entice customers to spend more money.

But these strategies are exacerbated online, where digital retailers can design their shopping experiences to appeal to the shopping behaviors of individual users. They can also use tactics that brick-and-mortar stores cannot, such as a "quick add-to cart" button, a one-time discount, a count-down clock or promote product reviews and ratings.

And, as the researchers found, online retailers can also add items to a customer's cart without their knowledge or consent. If the customer doesn't notice before they check out, they've just spent money they didn't intend to. The report gave an example of a flower delivery site adding a card to a user's cart in addition to a bouquet or floral arrangement.

And there are plenty other deceptive dark patterns at play.