In many of my projects, i create a java actions which just throws an exception (of my own custom exception class, usually with a single string argument (message)), to trigger custom exception handling.
Validation feedback does not trigger an exception. Even more important, you will notice validation feedback only if the microflow is called by a form, not when its called by a webservice.
Performing any "illegal" operation will trigger an error handler. Ie, the following microflow expression would trigger an error handler:
I'm sure there are other ways, I don't have an extensive list handy. Basically, anything that will cause java to throw an exception will also trigger an error handler in microflows.