Sorry for the wording,

Under artificial property I ment a property that is not appearing in
the component/page class as normal or abstract getter/setter.
Artificial property - property declared in the .jwc or .page files.
For example you might have a component called Basket
You will have a Basket.java -

public abstract class BasketComponent extends BaseComponent {

 public abstract boolean isProperty();

}

Then you will have a component descriptor

Basket.jwc

<component .... ... ... class="... ... BasketComponent">

<property name="property"/>

<property name="artificialProperty" initial-value="false"/>

</component>

As you can see component has two properties "property" and
"artificial". One of them appears in the java class as abstract
getter, another one do not appear anywhere except component
specification, however component specification may contain a OGNL
expression with all it's properties:
In this case "property || artificialProperty" which brings the error.

Links to the source code:

Java class: http://bookie.googlecode.com/svn/trunk/bookie-web/src/main/java/org/bookie/web/components/TabPanel.java
JWC descriptor:
http://bookie.googlecode.com/svn/trunk/bookie-web/src/main/webapp/WEB-INF/TabPanel.jwc

Error appearing in:

 <component id="link" type="DirectLink">
   <binding name="listener" value="listener:doClick"/>
   <binding name="parameters" value="blockId"/>
   <binding name="disabled" value="ognl: tabSelected || readonly"/>
   <binding name="async" value="ognl: !(tabSelected || readonly)"/>
 </component>

When evaluating

"ognl: tabSelected || readonly"