Fridge Horror: Just where did those shop keepers get that cursed armor from? And how many of those monsters you fight are other adventurers that have been polymorphed?
Fridge Logic: The description of each type of item before you identify it varies randomly from game to game. Combine this with a potion of drinking water and you end up with situations where drinking water glows, gives off smoke, is orange... just what do they put in the water over there? And not forgetting that if you mix two vials of it then it can explode in your face.
The wand of wishing, combined with being a high level sage and maximum luck. You can wish for extremely powerful weapons, armour so powerful that no enemy in the game can hit you, and you can make as many wishes as you want by using blessed scrolls of recharging... until the wand eventually explodes.
Get your constitution high enough and repeatedly drinking blessed potions of experience or eating dead wraiths can increase your hitpoints to ludicrous levels.
Writing itself turns into this at high levels, allowing you to rewrite scrolls into scrolls of blessing to improve weapons and armour, genocide many of the particularly nasty species, and write scrolls of time stop.
The scroll of switch bodies lets you switch into the body of the most powerful monster on the level. Moving your mind into the body of a draugr (the mightiest of the undead) is a common use for it; draugr are stronger than any human can possibly be while retaining the ability to use all equipment.
Some especially powerful items are only created once per game. Magicians have the ability to transform items in your pack into other items, potentially into these unique items. However the game doesn't actually register that the item has now been created, allowing you to obtain a second one if you can find one. Offsetting this somewhat, these items don't always have the characteristics of the genuine item; a red bag created in this way is not immune to further transformations, for example.
If your character dies, you can stumble across their possessions. Normally unique and quest items are stripped from your old character's inventory, but if you place these items in a red bag then the game will not remove them. This red bag is also not considered to be created by the game as above, allowing you to potentially find another one.
The bazaar is designed to make stealing impossible if you cannot kill the shopkeepers. Almost every form of escape will fail, including those that work everywhere else. However, if you use the orb of imprisonment on yourself then you can escape without paying.