Are you trying to pick up the Holy Water subweapon, perhaps? Picking it up
replaces whatever subweapon you're currently carrying (which explains why
the "potion" "changes into something else" when you pick it up), and
picking up the discarded subweapon discards the Holy Water (which explains
why the "something else" "changes back" to the "potion"). This behavior is
true of all of the subweapons in HoD.

HTH. HAND. :)