Hello,
I want to display a error message in a Message box in C#. So i tried the following :

Page.RegisterStartupScript("ScriptDescription", "<script type=\"text/javascript\"> alert('Please Enter a valid Comments');</script>");

-Sometimes the alert message is popping out and sometimes its not popping.

-Sometimes after it popps out, after i close the browser window, it gives me the following error:

"Microsoft Jscript Runtime Error:
Handler was not added through the SYS.UI.DOMEVENT.ADDHANDLER method."

-can anyone explain what went wrong?

It Seems to b fine.
u can try using
ClientScript.RegisterStartupScript(typeof(Page),"S criptDescription", "<script type=\"text/javascript\"> alert('MyMessage');</script>");

which browser u r using?