The theme: calculator mistakes on test day are almost never typing errors — they're reading errors. The calculator did its job; the student answered the wrong question, looked at the wrong part of the screen, or trusted a window that was hiding the answer.
1. Answering x when they asked for y (or x + y)
The intersection says (3, 2). The question asked for x + y. The trap is choice "3". Fix: re-read the final sentence of the question after tapping — every time.
2. The crossing is outside the window
Two lines that "never cross" usually cross off-screen — steep money and distance problems cross at values like (7, 266). Fix: if lines aren't parallel, the point exists; zoom out until it appears.
3. Wrong angle setting on trig
sin(30°) with the calculator in radians gives nonsense. Fix: wrench icon, top right — Degrees if the question uses °, Radians if it uses π. Check it before typing any trig.
4. Reading a rounded decimal instead of the exact fraction
0.8333... is 5/6. Fix: tap the fraction toggle beside the result to see the exact form the answer choices use.
5. Retyping instead of defining
Evaluating f at three inputs by retyping the expression three times triples the error surface. Fix: define f(x) once, then type f(2), f(5), f(−1).
6. Using the calculator for everything
Typing 12 × 5 into a graphing calculator costs more seconds than it saves. Fix: the calculator wins on equations, systems, unknowns, graphs, and data — mental math wins on arithmetic.
Error-proofing is practice, not memory
You don't fix these by reading a list once — you fix them by drilling until checking the question's final sentence and the window are automatic. That's what the practice arena trains, ten questions at a time.