When I start an ant-build from Jenkins it seems that Jenkins' own jar-Files become part of the build path. This can lead to all sorts of problems, for example this one:

In our build we use Checkstyle 6.8.1 which in turn uses Guava 18. Jenkins has Guava 11 in it's lib folder and the ant-build uses this one instead of the Guava 18 which is part of the builds classpath. This leads to an error because the Guava-API has incompatible changes:
java.lang.NoSuchMethodError: com.google.common.io.Closeables.closeQuietly(Ljava/io/InputStream

Jenkins own libraries should be strictly seperated and not become part of the classpath an ant-build from within Jenkins uses.