The Stacks project

Remark 35.14.6. With the assumptions (1) and $p$ smooth in Lemma 35.14.5 it is not automatically the case that $X \to Y$ is smooth. A counter example is $S = \mathop{\mathrm{Spec}}(k)$, $X = \mathop{\mathrm{Spec}}(k[s])$, $Y = \mathop{\mathrm{Spec}}(k[t])$ and $f$ given by $t \mapsto s^2$. But see also Lemma 35.14.7 for some information on the structure of $f$.