First Tip references a URL that no longer exists. The new url is https://github.github.com/training-kit/downloads/github-git-cheat-sheet.pdf.
Table 9.1 View is included on both the top and bottom rows of the table. The second "Views" row should be removed.
The hyperlinks in two locations should be localhost:3000 but instead currently points to localhost:300:
The URL in the paragraph beginning with "Restart your application" just before Quick check 10.1
The URL before the Warning just before Quick check 10.2
First Note, [Here, you're using the empty object to make it clear that you want to get all subscribers with no conditions attached.] is missing the are contraction.
There should be no space between mongoose and .Schema.types.
testSubscriber.courses.push(testCourse); should be