In the github-102 tutorial page for contributors there is a step #2 ("Pull in changes made by others") we instruct to them to run:
This will fail if they have not set an "upstream" repository. We didn't instruct them to do so in the github-101 tutorial. Also I think it's an easy mistake to think you already did that or to think git does it automatically.
I suggest the following addition, shown in red below, that links to the shortest github.com doc on how to do it.