based on what was shown on the show and what Troyzan posted on twitter
I believe Sarah forgot that the advantage was non transferable and was giving it to Cerie to use if she got voted out.
Sarah was so sure that Tai was loyal and Aubry was lying that she was willing to give Cerie the advantage if she was wrong.
Sarah read the advantage when she first found it and prob had not read what it said since then.
When Cerie tried to use it there was prob a discussion about it until Troyzan pointed out to read what it said.
If Sarah remembered it was non transferable and was testing Cerie, Troyzan would not need to get involved to point that out.
Edited (05/18/17 11:24 PM)