If $f'(t)$ cancels, you have a maximum in $x$. (Or minimum or inflection.)
IfBecause when $g'(t)$ cancels$f'=g'=0$, you have a maximum in $y$. (Or minimum or inflectionthe direction of the curve is no more defined.)
And if both cancel, you can have At such a singular pointpoint, because $x$ and $y$ momentarily "stop varying" and can restart in anotherthe curve could change direction abruptly.