"Yes!" on fat-fingering (or creaky sticky rubber buttons of older physical calculators that did not always make contact exactly once per intended depression), and in a way an argument to discussion raised by the article: during my school/uni days our generation passed the spectrum from avoiding calculators as an unholy evil, to getting the job done as you would at work - can use lecture notes (sometimes textbooks) and tools on exams since you'd do same in a lab or office. Not so much in the field work, on customer site, or when shopping though, where your head is all you have for facts and calculations.
Back to the point of calculators - bad buttons helped them lie, so in the mixed-acceptance approach we were allowed to use but trained to not trust them, and so mental maths was still practiced that one would sufficiently grasp the expected order of magnitude of the answer, and probably the digits that should be seen there, to worry about retrying the calculation in a timely fashion - ASAP.
Tired, undereducated or outright cheaty cashiers at shops or canteens around the campus also helped practice the quick mental maths, where you only have a couple of seconds to complete the task or forever hold your silence. Maybe that was done on purpose? ;)
Biting the hand that feeds IT
