Ok I think I found the issue. It happens only when you have shipping, on joomla 2.5 and with the CSS display mode configured for the buttons.
Change the code:
		if(shippings) {
			shippings = shippings.value.split(";");
			for(var i = 0; i < shippings.length; i++) {
				if(!varform.elements["hikashop_shipping_" + shippings[i] ] || !isSelected(varform.elements["hikashop_shipping_" + shippings[i] ])) {
					alert("'. JText::_('SELECT_SHIPPING',true).'");
					return false;
				}
			}
		}to
if(shippings) {
		var selectedShippings = new Array();
		if(shippings.value){
			selectedShippings.push(shippings.value.split(";"));
		}else{
			for(var a=0; a < shippings.length; a++){
				if(shippings[a].checked && shippings[a].value.length>0) selectedShippings.push(shippings[a].value);
			}
		}
		for(var i = 0; i < selectedShippings.length; i++) {
			if(!varform.elements["hikashop_shipping_" + selectedShippings[i] ] || !isSelected(varform.elements["hikashop_shipping_" + selectedShippings[i] ])) {
				alert("'. JText::_('SELECT_SHIPPING',true).'");
				return false;
			}
		}
	}in components/com_hikashop/views/checkout/view.html.php and that should fix the problem.