As a tech support person, I frequently get calls from people who have an unexpected suspicious message showing up in their web browser. The message might state that their computer is infected or needs an update. Sometimes a phone number is displayed and the person is urged to call the number immediately.
The video below shows how this problem happens and offers some suggestions on avoiding it, such as using ad-blocking software like Adblock Plus. The example provided in the video was recorded on 29 Feb 2020 from the New York Times website.