This post is inspired by an experience I had a few weeks ago, because I never noticed a calculator behave in an odd way. If I wasn't careful, I would not have realized it was giving me a different answer than what I wanted.
A lot of us, bored in class, used a calculator and did the game of typing [1] [+] [1] [=] and pressing [=] as much as we can to count up by one. To see how high you could get before it was time for recess or to move to the next class.
We understood the [=] to repeat the previous action, the +1. You could do this for doubling numbers too, just use
[1] [*] [2] [=] 2 [=] 4 [=] 8 [=] 16
Most calculators behaved the same for me. But a few weeks ago, I used a calculator as part of a standardized test (so not using your cell phone) that behaved differently.
This is expected behavior and what happened on this testing calculator:
[2] [+] [3] [=] 5 [=] 8 [=] 11 ; it keeps adding 3.
This is expected behavior and what happened on this testing calculator:
[3] [+] [2] [=] 5 [=] 7 [=] 9 ; it keeps adding 2.
This is expected behavior and what happened on this testing calculator:
[147.3] [/] [2] [=] 73.65 [=] 36.825 [=] 18.4125
Let's try to undo that... This is what happened on this testing calculator:
[18.4125] [*] [2] [=] 36.825 [=] 678.040313 [=] 12,484.4172
What happens on this calculator is the largest input is used, not the second input, when repeating the multiplication with the [=] key. Hitting [=] a second time did not do *2 again, it did *18.4125. This behavior was not true for Addition or Division, only for Multiplication. (I didn't test Subtraction.)
Someone not paying attention and doing something like 36*20*20 by doing [3][6][*][2][0][=][=] would have gotten the wrong answer of 25,960 on this calculator. (Correct answer is 14,400). If you don't know your maths and don't realize that a number looks like it's way off, you may not even realize the mistake.
tl;dr Calculators can behave differently from one another. They'll give the right answer, but they may use numbers you didn't intend to; this is a human error. You need to know how your calculator works.