This thread is questionable -– but I did have a similar-sounding issue recently with the iPhone calculator. I was doing a calculation where the answer was identical to the last number I entered before hitting "="; I don't recall the calculation, but for simplicity/illustration, let's say it was "88 – 44 = 44".
Due to poor UI in the caculator app, there is no indication that the calculation was performed, other than the number in the display "changing" to the answer — a change which does not happen in the scenario I'm describing. (Even on a cheap pocket calculator, the 44 would "flash" so you know the subtraction happened and the answer is also 44). And so when I hit "=" and nothing would change, I would hit "=" again, and get "0" this time (because it was then subtracting another 44 from 44).
I couldn't figure out what was going on, and kept trying it again and again (I was trying to do something under pressure, which didn't help...). Finally turned to a different calculator, and saw what was up.
All they need to do is add a "flash" visual feedback — like a cheap calculator from 30 years ago — to let you know the input was received. (This is how much better the stock Samsung calculator is: it shows the next answer in tiny numbers before you even hit "="; keeps a running tally of your calculations which you can scroll back through; etc. Do Apple programmers not use calculators?)