I've found two different situations that caused this error.
A second scenario was when I was setting focus to a RadNumericTextBox (the type of rad control may not matter). I couldn't simply set focus to the control, I had to tell the panel in which the numeric text box was in to set control to the text box.
txtToFocus = item.FindControl("txtQuantityOnHand") as RadNumericTextBox;
//txToFocus.Focus(); //this way causes client side issues.