Not sure what the article indents to show, those "surprises" are the expected behaviour of `append` on slices.
Yeah, this is a pretty bad article and telling the reader to figure out the "surprises" for themselves with no further explanation is just borderline trolling. Seriously, if you don't understand how slices work you shouldn't be claiming to "know Go".
In reality many programmers do not think much about how slices really work and got stuck with many gotchas from the mentioned quizzes.
If they're claiming to know the language when they actually don't then they are being fraudulent. How do these people manage to ship any real Go software without understanding basic language constructs? Are you next going to tell me that they don't understand interfaces or type embedding? I would appreciate your article more if you titled it something like "Learning about Go slices" instead of something so clearly click-bait.