Returning an incompatible type in a render phase method results in a following exception:
"The return value from a render phase event method was not compatible the expected return type of java.lang.Boolean. You should change the method to return the correct type."
The message of the exception is misleading because org.apache.tapestry5.internal.structure.RenderPhaseEventHandler also accepts RenderCommand and Renderable as return types. The message should be fixed.