Writing from Belgium here, and i found that those kind of things are unfortunately a common occurrence , both in standardized and teacher-made test.
Far more than the simplification errors (example 1 and 2 in the article, though both seems kinda gross) it's the second type of error that are far more annoying for a student passing a test. For the first type of errors, intelligent students can generally realize that the test is just asking for them to give the (flawed) answer that they have been taught, and they'll do so while making a mental note about its correctness in their head. But questions with multiple correct answer will confuse anyone who happens to recognize them, making them lose time in addition to risking a wrong answer. (btw multiple choices sucks, anyone with half a brain can get a passing grade without studying by applying some simple reasoning)
Now onto a personal experience (the real reason i wrote this post : I m still wanting to vent nearly 15 years later)
Elementary school, fourth grade, the question is counting the number of "rectangle" on a drawing. So I do the counting, then naively write down the answer I deem correct. A few days later the result come in, I happen to have a mistake on that question : the "correct" answer is one less than the one I had given, because some idiots writing standardized tests don't think a square is a rectangle (justification is about the same as the one given in this article, student not "smart" enough to realize it)
Nowadays, i m a university student, and its gotten a lot better once out of high school (well maybe it's just my faculty, applied sciences. You should see an economy teacher trying to explain second derivative to engineering students : he appears to understand what it does (i d hope so), but from the way is teaching it, either he's the worst teacher on earth (not confirmed on the rest of his subject) or some students in other sections must have a really hard time grasping anything about it), but i still sometimes go and check test and report card of my little sister, and i m really appalled by some the questions given. Most of the time the problem lie in badly formulated questions, but sometimes you'll find questions where it seems obvious the teacher has only a passing familiarity with the subject at hand (this is a plague in elementary school).
phew, rant off, i m out
Arithmetic is being able to count up to twenty without taking off your shoes. -- Mickey Mouse