From a bug report, drinking a potion of
polymorph which is wielded would trigger an "object lost" panic if hero
took on a form that was forced to drop its weapon. Once weapon/potion
got dropped, subsequent useup() of the potion was no longer operating on
an inventory object.
Unwield the potion at start of drinking, so that dropping doesn't
come into play. (If we ever introduce a monster form incapable of
holding inventory so drops everything, this will have to be revisited.)
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