Trend Micro researchers recently observed an increase in the volume of attacks targeting Twitter users in Japan that cause the victim's browser to crash.
Victims are lured to malicious sites via links in Twitter messages. When they click on the link, they're presented with a popup that tells them they won't be able to exit the page -- clicking on the 'OK' button simply prompts a new popup to appear.
This isn't actually dangerous -- nothing is downloaded to the victim's PC or mobile device -- but it can be incredibly annoying. On a PC, you can get out of the infinite loop by simply closing the window or tab that contains the popup. On a mobile phone, Katayuki suggests turning on airplane mode, restarting the phone, opening the browser, then closing the tab in question.
Katayuki notes that a similar method was used in 2011 to render a Japanese chat site unusable -- the attacker in that case was identitied by Japanese police and arrested.