The sun.instrument.TransformerManager.transform() method implementation, currently has this:

    try {
        transformedBytes = transformer.transform(   module,
                                                    loader,
                                                    classname,
                                                    classBeingRedefined,
                                                    protectionDomain,
                                                    bufferToUse);
    }
    catch (Throwable t) {
        // don't let any one transformer mess it up for the others.
        // This is where we need to put some logging. What should go here? FIXME
    }

As you will notice, anything thrown from the ClassFileTransformer's transform() method gets eaten up in the catch block without it ever being reported. There's even a FIXME code comment about it.

Addressing this may not be straightforward since java.lang.instrument.Instrument.addTransformer(...) notes (among other things):

> If a transformer throws an exception during execution, the JVM will still call the other registered transformers in order.

So it might need some additional thought on how/when these exceptions can be reported.