It's a javac bug - http://bugs.sun.com/view_bug.do?bug_id=6512707

Workaround is to add <proc>none</proc> to the maven compiler plugin.  I'm sure I won't be the last person to have this problem :-/

<build>
    <plugins>
        <plugin>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>2.3.2</version>
            <executions>
                <execution>
                    <id>default-compile</id>
                    <phase>compile</phase>
                    <goals>
                        <goal>compile</goal>
                    </goals>
                </execution>
            </executions>
            <configuration>
                    <!-- proc none to fix http://bugs.sun.com/view_bug.do?bug_id=6512707 -->
                <proc>none</proc>
                <debug>true</debug>
                <source>1.6</source>
                <target>1.6</target>
            </configuration>
        </plugin>    
    </plugins>
</build>

Since I never attached the source, the trigger is to have an annotation class like this:

public @interface A {
    static class None extends Throwable { private None() {} }
    Class<? extends Throwable> expected() default None.class;
}

With annotation processing enabled, the compiler thinks None.class is not the same type as ? extends Throwable.

I think Jan knows a workaround and actually if you put openide-filesystems.jar on classpath, your problem may disappear...

-proc:none is a fine workaround if no annotation processing is needed. If AP is needed, then the problem can be workarounded using:
http://hg.netbeans.org/main-silver/file/tip/openide.filesystems/src/org/netbeans/modules/openide/filesystems/CleaningAnnotationProcessor.java
http://hg.netbeans.org/main-silver/file/tip/openide.filesystems/src/org/netbeans/modules/openide/filesystems/CleaningAnnotationProcessorImpl.java
(ideally, CAP should be the very last AP to run as it strips the attribution info which the older javacs forget to clean). Using a recent JDK7 javac should probably resolve the problem as well.