What does "><img src=x onerror=confirm(1);> do?

What does "><img src=x onerror=confirm(1);> do?

  
Hi, I have seen that in the forum lately.. alot..  WHat does that do?
Ouch, that handle actually displays an alert in the browser. Bad! OutSystems, fix that please! (Cudos to Ben for being so creative...)

@Ben: where do you see that?