The simplest thing to do is to use a label control and normally it will
not be visible. I also like to set the font color to red.

<asp:Label id=errMsg runat=server fore-color=red visible=false />

Then in the code, when I detect the error,

If(thatErrorThing)

{

errMsg.Text = "Some type of message";

errMsg.Visible = true;

return;

}

in the page load event, always do

errMsg.Text = "";

errMsg.Visible = false;