= Solution
Three different omissions produce concrete erroneous <STRIPS> plans. Here a route means the corresponding successive $\operatorname{Move}$ actions with the indicated first argument.
\b[No lethal-entry condition.] Move the agent along $s_{11},s_{21},s_{22},s_{23},s_{13}$, take the gold at $s_{13}$, and reverse the route. This nine-action <STRIPS> plan satisfies every supplied <precondition> and the final <goal test>, without firing. Physically it fails on entering the live monster’s room: the supplied <search transition model> neither forbids the entry nor records death. The omitted pit cell happens to be disconnected in the supplied adjacency relation, so this particular instance already excludes pit entry; that accidental protection does not encode the monster hazard.
\b[Supposedly stationary objects can move themselves.] The five-action <STRIPS> plan
$$
\begin{aligned}
&\operatorname{Move}(g,s_{13},s_{23}),\quad\operatorname{Move}(g,s_{23},s_{22}),\\
&\operatorname{Move}(g,s_{22},s_{21}),\quad\operatorname{Move}(g,s_{21},s_{11}),\\
&\operatorname{Take}(a,g,s_{11})
\end{aligned}
$$
leaves the agent at the exit and brings it the gold. The formal <preconditions> hold because $x$ in $\operatorname{Move}(x,y,z)$ has no actor restriction. Gold cannot actually propel itself. The same flaw permits moving the supposedly stationary monster.
\b[Unrestricted pickup permits impossible inventory.] Follow the first example’s outward route, but insert $\operatorname{Take}(a,w,s_{23})$ immediately after reaching $s_{23}$. Continue to $s_{13}$, take the gold and return. This ten-action <STRIPS> plan ends with both $\operatorname{have}(a,w)$ and $\operatorname{have}(a,g)$ and satisfies the supplied goal. Its entrance also exposes the first flaw, but the additional, distinct defect is that pickup treats the live monster as a collectible object and deletes its location. A movement safety check alone is not a specification of which objects are collectible. The shot schema likewise lacks restrictions to the designated shooter, projectile and victim; fixing the action roles together prevents these type errors.
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