If a number is in the denominator of a rational expression that itself is the denominator of another rational expression, such that the number causes the latter denominator to be undefined, then isn't the entire expression undefined?
For example, isn't $$\frac{\frac{x^2-3x-4}{-3x-15}}{\frac{x^2-16}{x^2-x-30}}=\frac{\frac{(x-4)(x+1)}{-3(x+5)}}{\frac{(x+4)(x-4)}{(x-6)(x+5)}}$$ undefined at $x=6\;?$ $$\frac{\frac{x^2-3x-4}{-3x-15}}{\frac{x^2-16}{x^2-x-30}} \implies\quad\frac{(x-4)(x+1)(x-6)(x+5)}{-3(x+5)(x+4)(x-4)},x\neq{-5},{-4},4,6\quad?$$
If so, why do my scientific calculator and Desmos compute the expression as zero at $x=6\;?$