Depending on your income, there’s a good chance you can file your taxes online for free. Over the years, however, a handful of tax prep companies have misled millions of consumers by hiding this option on their websites.

That’s according to a recent report from New York’s Department of Financial Services, as well as a series of reports from ProPublica. If you’ve used any of these services—TurboTax, H&R Block, TaxSlayer, TaxHawk, and Drake Enterprises are the five named in the report—you may have been tricked into paying for a service you didn’t need to buy.

It’s an extreme example of what UX experts call “dark patterns,” a term that consultant Harry Brignull coined and is used to describe user interface design that “makes it very easy to get into a situation but very hard to get out.” A popular example of a dark pattern: an email popup that prompts you to sign up with either a “Yes, I’m interested” or a passive-aggressive “No, I don’t like saving money.”

An endless stream of brands, products, and companies compete for your attention online. While some of them take the time to gain your readership and trust, others rely on manipulating your behavior by using your own psychology against you. Some of these practices are benign, while others, like the tax prep example, can cost consumers money. One study found that 11 percent of the 11,000 retail websites analyzed used some form of a dark pattern on their interface. And the more popular the website, the more likely it was to use dark patterns.

This is an especially problematic practice in the financial services industry, which is already criticized for using complicated jargon that consumers find confusing and often predatory. For example, a recent report found that many banking sites are harder to read than Moby-Dick—which doesn’t sound that bad if you’re in eighth-grade English, but when you’re simply trying to pay an online bill, it’s a frustrating problem. Fifty-eight percent of this content is too complex for the typical consumer, the report confirmed.

“It is crucial that people understand the products they are using,” says Linda Sherry, director of national priorities for advocacy group Consumer-Action. “While I wouldn’t outright call a lot of the practices deceptive, consumers are often led by their noses when they sign up for products and services online,” she says. “When they come to a new website with the aim of doing business, they obviously are steered into what the bank or other company wants them to see or do.” Sometimes the patterns are subtle—the websites might use clever images, text placement, or bright colors that steer consumers into a particular action, account, or service, Sherry says.

Using bright colors to entice website visitors is harmless enough, but other types of dark patterns can trick you into handing over your data, signing up for emails you don’t want, or, worse, paying for services you don’t intend to buy. Pair the already problematic way financial services reach out to consumers with the added problem of manipulative UX, and it’s easy to see why consumers need to be extra careful when navigating the websites and apps of any financial product. “I think they’re all employing techniques to make you give up,” Sherry says. “For example, my bank used to have a secure message area as part of online banking. That disappeared and now you have to call. The terrible music and the constant sales pitches that you hear while waiting are annoying and frustrating.”

It’s not just banks, of course. Revolut, a money management app, has also been criticized for making it difficult to opt out of their targeted marketing. This is a form of dark pattern called misdirection—consumers are made to believe they’re selecting one option when they’re sneakily being misdirected to another. Robinhood, the investing app, uses a gamified interface to make investing seem easier than it is, encouraging its users to trade frequently, which most personal finance experts do not recommend for long-term investing. Without properly understanding the risk, many consumers have been steered into costly investing decisions.