Run the following program on JDK 6:

---%<---
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileWriter;
import java.io.IOException;
import java.io.Writer;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.Set;
import javax.annotation.processing.AbstractProcessor;
import javax.annotation.processing.RoundEnvironment;
import javax.annotation.processing.SupportedAnnotationTypes;
import javax.annotation.processing.SupportedSourceVersion;
import javax.lang.model.SourceVersion;
import javax.lang.model.element.TypeElement;
import javax.tools.Diagnostic.Kind;
import javax.tools.JavaCompiler;
import javax.tools.JavaCompiler.CompilationTask;
import javax.tools.StandardLocation;
import javax.tools.ToolProvider;
public class Demo {
    public static void main(String[] args) throws Exception {
        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
        System.err.println("using " + compiler.getClass() + " from " + compiler.getClass().getProtectionDomain().getCodeSource());
        String tmp = System.getProperty("java.io.tmpdir");
        for (File kid : new File(tmp, "p").listFiles()) {
            kid.delete();
        }
        File input = new File(tmp, "X.java");
        Writer w = new FileWriter(input);
        w.write("package p; class X {{p.C.first();}}");
        w.close();
        List<String> opts = Arrays.asList("-s", tmp, "-d", tmp);
        CompilationTask task = compiler.getTask(null, null, null, opts, null, compiler.getStandardFileManager(null, null, null).getJavaFileObjects(input));
        task.setProcessors(Collections.singleton(new Proc("first")));
        System.err.println("success? " + task.call());
        w = new FileWriter(input);
        w.write("package p; class X {{p.C.second();}}");
        w.close();
        task = compiler.getTask(null, null, null, opts, null, compiler.getStandardFileManager(null, null, null).getJavaFileObjects(input));
        task.setProcessors(Collections.singleton(new Proc("second")));
        System.err.println("success? " + task.call());
        task = compiler.getTask(null, null, null, opts, null, compiler.getStandardFileManager(null, null, null).getJavaFileObjects(input));
        task.setProcessors(Collections.singleton(new Proc("second")));
        System.err.println("success? " + task.call());
    }
    @SupportedAnnotationTypes("*")
    @SupportedSourceVersion(SourceVersion.RELEASE_6)
    private static class Proc extends AbstractProcessor {
        final String m;
        Proc(String m) {
            this.m = m;
        }
        int count;
        @Override public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
            if (roundEnv.processingOver() || count++ > 0) {
                return false;
            }
            System.err.println("running Proc");
            try {
                processingEnv.getMessager().printMessage(Kind.NOTE, "found previous content of length " +
                        processingEnv.getFiler().getResource(StandardLocation.SOURCE_OUTPUT, "p", "C.java").getCharContent(false).length());
            } catch (FileNotFoundException x) {
                processingEnv.getMessager().printMessage(Kind.NOTE, "not previously there");
            } catch (IOException x) {
                processingEnv.getMessager().printMessage(Kind.ERROR, "while reading: " + x);
            }
            try {
                Writer w = processingEnv.getFiler().createSourceFile("p.C").openWriter();
                w.write("package p; public class C {public static void " + m + "() {}}");
                w.close();
                processingEnv.getMessager().printMessage(Kind.NOTE, "wrote new content");
            } catch (IOException x) {
                processingEnv.getMessager().printMessage(Kind.ERROR, "while writing: " + x);
            }
            return true;
        }
    }
    private Demo() {}
}
---%<---

JDK 6 compiles successfully, as expected:

---%<---
using class com.sun.tools.javac.api.JavacTool from (file:/.../jdk1.6.0_26/lib/tools.jar <no signer certificates>)
running Proc
Note: not previously there
Note: wrote new content
success? true
running Proc
Note: found previous content of length 57
Note: wrote new content
success? true
running Proc
Note: found previous content of length 58
Note: wrote new content
success? true
---%<---

But JDK 7 fails:

---%<---
using class com.sun.tools.javac.api.JavacTool from (file:/.../jdk1.7.0-b146/lib/tools.jar <no signer certificates>)
running Proc
warning: Supported source version 'RELEASE_6' from annotation processor 'Demo$Proc' less than -source '1.7'
Note: not previously there
Note: wrote new content
1 warning
success? true
running Proc
warning: Supported source version 'RELEASE_6' from annotation processor 'Demo$Proc' less than -source '1.7'
Note: found previous content of length 57
Note: wrote new content
/tmp/X.java:1: error: cannot find symbol
package p; class X {{p.C.second();}}
                        ^
  symbol:   method second()
  location: class C
1 error
1 warning
success? false
running Proc
warning: Supported source version 'RELEASE_6' from annotation processor 'Demo$Proc' less than -source '1.7'
Note: found previous content of length 58
Note: wrote new content
1 warning
success? true
---%<---

While the processor regenerated C.java with a new signature, javac continued to use the prior content for compiling X.java.