I just took the 1Z0-809 (and passed).
There was an elaborate question on the Files.write(...) method. This was never mentioned in any of the books or tests I used for preparation.
Also there were quiet a lot of questions on FunctionalInterface and Streams. You have to look (and maybe experiment with) the Functional Interfaces for Primitives too.
And there was a question on ForkJoin which expected you to debug the damned program (I hate that stuff on examinations...)