This post was inspired by the Trousers of the Crab magic item over on the Ancient Vaults & Eldritch Secrets blog, so you'll want to go read that one first to understand this one. My sincere thanks to bät for something to riff on, as well as years of entertaining posts to read.
Trousers of Carcinization
Roughly 1 in 20 Trousers of the Crab are actually cursed due to a botched enchantments, producing Trousers of Carcinization. These function identically to the normal version with two exceptions:
1) Any humanoid creature coming in contact with the transformed wearer must save vs. magic or undergo an identical transformation that lasts for 1-6 hours. This probably has dire effects on one's wardrobe, which won't adapt to the sudden change the way Trousers of the Crab do. This curse is contagious and can be passed on by victims through contact in the same way they were originally infected.
2) Upon removing the enchanted trousers (or at the end of the curse's duration for other victims) there is a 10% chance that the transformation spontaneously recurs 1-6 hours later, this time becoming permanent but thankfully non-contagious. Only a Remove Curse or similar magic can end the curse once this occurs.
Further Notes
Rumors persist that even with properly-made Trousers the enchantment may spread to other items of apparel over time, making it dangerous to keep these things in a wardrobe, closet or backpack unless you enjoy surprise transformations when dressing. Such tainted clothing results in crab-form mutations appropriate to the body part they're meant to be worn on, resulting in effects ranging from inconvenient to horrific.
Cestus/Gauntlet/Glove of the Crab
It's possible that these rumors may be the result of encounters a different magic item, the Glove (or sometimes Gauntlet or Cestus) of the Crab. These are popular weapons/tools in some circles, albeit not without their drawbacks.
When worn by a humanoid creature, this magic item transforms the forearm and hand into a large crab claw until removed. This claw can be used for fighting (dealing damage as a broadsword) and each claw improves your armor class or parrying ability by one (or 5% for d100 systems). The claws are useless for fine manipulation, but can exert great crushing force (treat as 125% of the wearer's normal strength for that purpose only) and effortlessly retain a strong grip for hours.
These are usually found singly, but if a pair are found (10% chance) they function only when both are worn at once. This allows an extra claw attack and improves defenses as above, but the wearer will require assistance (or bare feet and some skill as a contortionist) to remove the gloves.
