SciCalc

A full scientific calculator — trig, logs, powers, memory and history, all on-device.

The two settings that produce most wrong answers are the angle mode and operator precedence. Trigonometric functions default to radians in most systems and to degrees in most people's heads, and the result of using the wrong one is plausible rather than obviously broken.

A visible history is the underrated feature. A calculation done in six steps has six chances to mistype, and without a record the only way to check is to do the whole thing again and hope you make the same mistakes or none. Being able to read back what was actually entered turns verification into reading rather than repetition.