The following used to compile on JDK 6 and no longer compiles on JDK 7 (b40).
javac 1.7.0-ea
java version "1.7.0-ea"
Java(TM) SE Runtime Environment (build 1.7.0-ea-b40)
Java HotSpot(TM) Server VM (build 14.0-b07, mixed mode)
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public class Node { ... }

public class AbstractNode extends Node {
    private Image findIcon(int a,int b) { ... }
    ...
}
public class DataNode extends AbstractNode { ... }

public class DataFolder {
   static Image findIcon(int a, String s, String s2) { ... }
   public class FolderNode extends DataNode {
        public Image getOpenedIcon (int type) {
              ...
              Image img = findIcon(someint, somestring, someotherstring);
              ...
        }
   }
}

netbeans_platform/openide/loaders/src/org/openide/loaders/DataFolder.java:1123: method findIcon in class org.openide.nodes.AbstractNode cannot be applied to given types
    [javac] required: int,int
    [javac] found: int,java.lang.String,java.lang.String
    [javac] img = findIcon(0, "Nb.Explorer.Folder.icon", "Tree.closedIcon"); // NOI18N
    [javac] ^

I have attached a smaller test cases which shows this behaviour:
See Main.java and NewClass.java attached.
It compiles with JDK 6 compiler, and fails with JDK 7.