java.lang.LinkageError

java.lang.LinkageError: loading constraint violation: loader "org/eclipse/osgi/internal/baseadaptor/DefaultClassLoader@621b621b" previously initiated loading for a different type with name "javax/wsdl/factory/WSDLFactory" defined by loader "org/eclipse/osgi/internal/baseadaptor/DefaultClassLoader@4c814c81

This is the best explanation I've found so far:

LinkageError is what you'll get in a classic case where you have a class C loaded by more than one classloader and those classes are being used together in the same code (compared, cast, etc).