Drive-by downloads may happen when visiting a website, opening an e-mail attachment or clicking a link, or clicking on a deceptive pop-up window: by clicking on the window in the mistaken belief that, for example, an error report from the computer's operating system itself is being acknowledged or a seemingly innocuous advertisement pop-up is being dismissed. In such cases, the "supplier" may claim that the user "consented" to the download, although the user was in fact unaware of having started an unwanted or malicious software download. Similarly if a person is visiting a site with malicious content, the person may become victim to a drive-by download attack. That is, the malicious content may be able to exploit vulnerabilities in the browser or plugins to run malicious code without the user's knowledge.