Back button hijacking occurs when a site prevents users from “using their back button to immediately get back to the page they came from.” Users are instead sent to “pages they never visited before, be presented with unsolicited recommendations or ads, or are otherwise just prevented from normally browsing the web.”

As Google notes, this breaks the “fundamental expectation” of how a browser’s back button should work. Besides breaking browser functionality, it “breaks the expected user journey” and “results in user frustration.”