This tip is intended to tell our readers that they should expect the unexpected in the test questions. For instance one student asked why the technique of "Activity on Arrow" was a question he got on his exam, even though this technique had been removed from The PMBOK Guide. My answer was as follows:
The PMBOK Guide is a GUIDE. It doesn't contain 100% of the concepts and tools that project managers around the world use. As such both AON and AOA are still being used. Also, PMI clearly says that the exam will also test you on concepts that are not necessarily found in the PMBOK Guide (but they cleverly omit telling us where we could find these...).
So what is the consequence of all of this? Don't be surprised to find questions on the test that are about concepts that are not (or no longer) on the PMP Exam. But don't let that worry you too much. Simply accept that this might be happening, read lessons learned from others so that you can see what might be on the test and do your best as you prepare for the exam.
Generally: Write this tip in such a way that the students understand that there may be things on the exam that they never heard of, or had expected not to be tested on. What to do in such a moment? Try your best in answering the question.