Seems some JRuby exceptions are (misusing) Throwable.getCause, by returning "this" instead null. Throwable.getCause should return some other instance, or null, never this. Moreover, this really smells like some method override in some Throwable that does not do like all throwables do:

public synchronized Throwable getCause() {
    return (cause==this ? null : cause);
}

For original report, see this:
https://issues.apache.org/jira/browse/MNG-7846