in the onboarding there is a logic in the clean pom to leave the folowing files:

<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-clean-plugin</artifactId>
<version>2.5</version>
<configuration>
<excludeDefaultDirectories>true</excludeDefaultDirectories>
<filesets>
<fileset>
<directory>${project.build.directory}</directory>
<excludes>
<exclude>test-classes/**</exclude>
<exclude>pmd/**</exclude>
<exclude>maven-status/**</exclude>
</excludes>
</fileset>
</filesets>
</configuration>
</plugin>

as a result, consecutive executions will file because not all files are deleted.