Just found VERY hard bug. The problem appears only in IE6, but some projects still support this browser. If you are using <a href="javascript:void(0);" >Button</a>

And attached OnClick event. You can encounter some really weird things (in event hadler, it works but some strange way). For me - I had to change src of some img tag. But after this no reload happened. Then I've used href="#" the problem was solved. Somebody knows more about these problems?