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Added in response to comment below: E.g. if $E$ is an elliptic curve over $\mathbb Q_p$ with good reduction, and $P$ is an $E$-torsor with no $\mathbb Q_p$-rational point, then $P$ will not have good reduction. It will not have semi-stable reduction either, since it has potentially good reduction (it obtains a rational point over some extension of $\mathbb Q_p$, and hence becomes isomorphic to $E$ over that same extension). The etale cohomology ($\ell$-adic or $p$-adic) of $P$ coincides with that of $E$, and so $P$ is an example of a variety over $\mathbb Q$ Q_p$with semistable (indeed crystalline) Galois action on its$p$-adic etale cohomology, which does not have semistable reduction. 2 added 699 characters in body Just to elaborate on Francois Brunault's answer: it is not true in general that having semistable etale cohomology implies having semistable reduction (just as it is not true that having crystalline etale cohomology implies having good reduction). So the implication only goes in one direction. Added in response to comment below: E.g. if$E$is an elliptic curve over$\mathbb Q_p$with good reduction, and$P$is an$E$-torsor with no$\mathbb Q_p$-rational point, then$P$will not have good reduction. It will not have semi-stable reduction either, since it has potentially good reduction (it obtains a rational point over some extension of$\mathbb Q_p$, and hence becomes isomorphic to$E$over that same extension). The etale cohomology ($\ell$-adic or$p$-adic) of$P$coincides with that of$E$, and so$P$is an example of a variety over$\mathbb Q$with semistable (indeed crystalline) Galois action on its$p\$-adic etale cohomology, which does not have semistable reduction.