Sometimes a white lie or two can help a user test along. I've long advocated a couple of these when running user testing training. I now have a few more candidates to add to the list.
This isn't really about lying, of course. It's about maintaining your participants' self esteem and making the most of the time you have with them.
Author Lisa Duddington also looks behind the lines she's feeding you - why and when they can be useful and also highlights the psychological research ethics to always bear in mind.
5 Useful Lies to Tell User Research Participants - article by Lisa Duddington