I have HPLIP 3.9.8 (RC) and I have set up the HP PhotoSmart C8180 with the hpcups driver. I select Duplex (long edge) and the A4 Duplex paper size. Then I print the test page from system-config-printer. The test page (it is an original CUPS test page and not a Ubuntu test page as the selected paper size is not simply named "A4") gets printed and in the end it gets pulled in to turn it over and then it sits there inside the duplex unit for several minutes, the job is still marked as "processing" in the job viewer. The printer says "Printing" on its display. After some minutes the sheet gets ejected, the printer still says "Printing" and the job is still in the queue "Processing". The driver seems to have fallen into an infinite loop. After some minutes I cancel the job in the job viewer. The job disappears and the printer says "Printing canceled" but does not return to its main menu screen. I have top turn off the printer and turn it on again. Symptoms are the same as in bug #184409.
One strange thing is also that with this 1-page job it says "processing page 3" in the Properties dialog of system- config- printer, after the printer has pulled back the paper.