This thing is that when you design your method in a procedural way all you input parameters are optional by default and are populated with default values if you don't specify them explicitly. Assume you have a method with the following signature:
[OperationContract]
void TestMethod(string param1, int param2);
You will get the following WSDL for it:
<xs:element name="TestMethod">
<xs:complexType>
<xs:sequence>
<xs:element minOccurs="0" name="param1" nillable="true" type="xs:string" />
<xs:element minOccurs="0" name="param2" type="xs:int" />
</xs:sequence>
</xs:complexType>
</xs:element>
As you can see minOccurs attribute has 0 value which means the element is optional. So this is not surprising that your method works even after you added a new parameter.
If you want to avoid this behavior try to design your contracts in a message way by using MessageContract or at least wrapping all you parameters in a container class. And specify explicitly which parameter is required and if it allows default value via DataMember attribute.
Hope it helps!