A few correct derivatives will not be recognized unless you enter them the "correct" way. For example, if you simplify the function sqrt(x²) to x, and enter the derivative as 1, it would (most of the time) be ruled incorrect. This is because sqrt(x²) = |x|, and the derivative of |x| is ±1 (specifically, +1 when x>0, and -1 when x<0). If you enter the derivative as abs(x)/x, or as x/sqrt(x^2), it will be counted as correct.

To check if your answer is correct, the computer finds the exact derivative. If your function and the exact derivative have the same output value at 5 randomly selected x values between –8 and +8, it is judged to be the correct answer. There is a small chance that the actual derivative might be undefined for all x in (–8,8). Other than this potential problem, I know of no other issues, but I have not tested this page to the point that I can guarantee it will never fail.