The problem (which may or may not be a bug) is demonstrated by the starred line of the following program:

public class SillyInstantiator<T> {
    public <U> U newObj(Class<U> klass) throws Exception {
        return klass.newInstance();
    }

    public static void main(String[] args) throws Exception {
        SillyInstantiator<Integer> si = new SillyInstantiator<Integer>();
        StringBuffer i1 = si.newObj(StringBuffer.class);
        SillyInstantiator rawSi = si;
        StringBuffer i2 = rawSi.newObj(StringBuffer.class); // *
    }
}

SillyInstantiator.java:10: incompatible types
found : java.lang.Object
required: java.lang.StringBuffer
        StringBuffer i2 = rawSi.newObj(StringBuffer.class);
                                      ^
Note: SillyInstantiator.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
1 error