Before entering a long expression in the Scientific Calculator, verify a few known values. In degree mode, sin(30) should be approximately 0.5. In radian mode, sin(π/6) should give the same value. A different answer may mean the angle mode is wrong.
Reproduce the operation order
Compare 2 + 3 × 4 = 14 with (2 + 3) × 4 = 20. Use parentheses around a negative base: (-2)^2 is 4. A parser may interpret -2^2 as -(2^2). When notation is ambiguous, split the expression into explicit steps.
Check that log(100) means the base-10 logarithm and returns 2. ln denotes the natural logarithm. Keep the expression with the saved result so another person can reproduce it.
Read precision honestly
JavaScript numbers have finite precision. Very large factorials overflow, and subtraction of nearly equal numbers can lose precision. Keep extra digits during intermediate steps and round only when justified by the input. A displayed last digit is not a proof of exactness. See JavaScript Number.