When I go in the equip menu, my hero is equiped with a potion whereas I've never equiped him before

If you have not specified a default weapon for your hero, then your hero will use item number zero for a default weapon, even if item 0 is a non-weapon item like a potion.

To correct this, just go into the hero editor, and then into the "Appearance and Misc" menu, and choose a default weapon.

To prevent this error from happenning it is recommended not use item (or to give it the name unequiped)