So if you've decided to use syntax such as this
myDropDownListBox.Items[0].Selected = true;
or
ListItem foo = myDropDownListBox.Items.FindByValue(”foo”);
foo.Selected = true;
you will get the 'A DropDownList cannot have multiple items selected' error.
Seems to me that if the code sets the Selected attribute on an item in a list which by definition can only have one item selected, that the ListItem instance would set the SelectedIndex automatically for me. Seems like a no-brainer. I guess Microsoft found some sort of use for this error being thrown from a DropDownList control - just don't ask me what that could be.
I guess I'd rather see that the DropDownList have DropDownList items if the Selected property was pretty much useless and will only cause errors if used in a DropDownList.