Applying "x!" to a non-integer positive number (e. g. 2.5!) gives an incorrect result, the factorial of the integer part (in this case 2). Factorials of non-integer numbers must either be calculated using the gamma function, n! = Γ(n+1), or throw an error message if this is impossible to implement.
https://en.wikipedia.org/wiki/Gamma_function
PS, applying x! to an integer negative number must always throw an error, the gamma function (therefore, the factorial as well) is undefined at these points.