Degrees Against Radians
This one wrecks whole trigonometry papers. Sine of 30 is 0.5 in degrees and about -0.988 in radians, and the calculator will not warn you which mode it is in. Check the display indicator before the first trig question rather than after the last one.
If an answer is wildly off and the working looks right, mode is the first thing to check.
The Minus Sign That Is Not a Subtraction
Scientific calculators separate the subtract key from the negative key. Entering -3 squared with the subtract key gives -9, because it reads as the negative of 3 squared. Using the negative key gives 9, because it squares negative three.
Both are correct interpretations of different expressions, which is exactly why it is dangerous.
Brackets You Assumed Were There
Typing 6 divided by 2 plus 4 gets you 7, because the calculator divides by 2 first. If the fraction bar on paper covered the whole of 2 plus 4, the answer is 1. The calculator cannot see your paper. Type the brackets.
Rounding on the Way Through
Rounding an intermediate value and then continuing bakes the error into everything after it. Keep full precision until the end, or use the answer memory rather than retyping a shortened number.
Trusting the Screen Over an Estimate
Before you press equals, know roughly what to expect. If 19.8 times 4.1 should be about 80 and the screen says 8.1, you have caught a typo that would otherwise have been copied straight onto the answer line.
A rough estimate takes two seconds and catches the majority of input errors.
Leaving Memory Loaded
A value left in memory or a constant still active from the previous question quietly contaminates the next one. Clear between questions; it costs one keypress.
