Rounding too early
Rounding an intermediate answer before using it in the next step compounds error through the rest of the question. Keep full decimal values in your calculator (or write 4+ decimal places) until the very final answer, then round once.
Mode mistakes
Leaving your calculator in the wrong angle mode (degrees vs. radians) silently produces confidently wrong trig answers with no error message. Check your mode at the start of every trig question, every time — not just once at the start of the exam.
Not showing the calculation
Typing an expression straight into your calculator and writing only the final number means a marker can't award method marks if that number is wrong. Write the formula and substitution on paper first, then use the calculator just to compute — this protects your method marks even if you mistype something.