These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
The Grade-A Mushroom enemy from the first game. Attacking them—especially with a sword equipped—has a high chance of duplicating the Mushroom, causing more enemy Mushrooms to appear. Not only that, but they have a powder attack that can cause both Confusion—which prevents you from moving or attacking straight, but you can still throw straight and use talismans—and Poison—which prevents natural healing. There is nothing stopping them from spouting powder more than once. Combined with the fact that there is also an enemy in the same dungeon as them—Ittan Nylon—that can give you the Seal status effect, preventing you from using talismans to help yourself, the Grade-A Mushroom becomes an enemy to fear.
The butterfly enemy has the ability to lower your level, and there is nothing preventing them from doing it more than once.
In the first game, the high-speed enemies. They can move twice in a single turn, as well as move and attack in a turn. Luckily they can't attack twice in a single turn.
The Talis line of enemies, specifically the Talisouls and Talispecters. These enemies can use a special attack that sticks a Suitai talisman to your equipment, which lowers the equipment's Attack and Defense slightly. There is nothing stopping them from doing this more than once. They can use this move from a ranged distance. Luckily, not being very smart enemies, sometimes the Talis enemies throw the Suitai talisman onto other enemies, lowering their level and making them less powerful and easier to defeat.
Les Yay: Izuna and Shino, mostly in promotionalart for the series. Izuna and Shizune as well, both in promotional art and their team up move.
That One Side Quest: Completing your gallery entries, which requires film to take the photos of one-time only characters such bosses, and in the sequel, Tsuki before finding out who she really is and pre-unmasked Mitsumoto. Film is also somewhat scarce to find in dungeons.