In answer to my question
I was pointed to this thread on M.SE as a duplicate:
I think that this is an excellent example for a general discussion: On the surface, the question are practically the same "Look at this example, give me similar examples.", but on a first view of the M.SE I get the impression that it would be hard to find examples there that are usable for a course.
It looks more like a collections of anecdotes "Hey, I had a student who thought that since $x^2+y^2=1$ gives a circle, $x^2+\frac 12 y^2= 1$ gives a semi-circle." which can be fun, but not really useful to present to students.
I am really not that invested in my particular question if you do not like it, but the general question is:
At what point does the expectation of a very different kind of answer here make a very similar question not a duplicate to M.SE?