I had a similar problem in NetBeans recently and found a fix now. The problem first started when I added an EntityClass and had NetBeans automatically add the persistence unit, etc.
The problem seems to be caused by the persistence unit adding files to your projects META-INF folder which will use a different technique to calculate the jars signature, or something like that.
To remove these files, add this after the exec-maven-plugin in your pom:
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>truezip-maven-plugin</artifactId>
<version>1.2</version>
<executions>
<execution>
<id>remove-RSA-and-SF-files</id>
<goals>
<goal>remove</goal>
</goals>
<phase>package</phase>
<configuration>
<fileset>
<directory>${archive}/META-INF</directory>
<includes>
<include>**/*.RSA</include>
<include>**/*.SF</include>
<include>**/*.DSA</include>
</includes>
</fileset>
</configuration>
</execution>
</executions>
</plugin>
you also need to add the archive variable, which points to your .jar file:
<archive>target/projName-1.0-SNAPSHOT.jar</archive>
and finally add to your run task in nbactions.xml:
<goal>org.codehaus.mojo:truezip-maven-plugin:1.2:remove</goal>
just before:
<goal>org.codehaus.mojo:exec-maven-plugin:1.2.1:exec</goal>
Hope that helps!