Polymorphing a dragon corpse creates a random object made of dragonhide.

When you attempt to polymorph an item that has polymorph magic (wand/spell/potion of polymorph, tin of meat that may cause a polymorph when eaten, etc) it explodes and creates one or more monsters affiliated with polymorph magic, such as polymorph vortices (see YANI #3669), genetic engineers, or similar.

Polymorph vortices, a monster which has a passive polymorph attack (i.e. hitting it in melee polymorphs the attacker; hitting it with missiles polymorphs the missiles) and once per turn polymorphs either one random item from its inventory, a monster it has engulfed, or a random item from that monster’s inventory. Player-style magic resistance prevents the effects to a creature or its possessions that would otherwise be affected.

Scrolls can be “corrupted” or “misspelled”. This basically acts as a toggle on whether the confused behavior of the scroll activates: reading a corrupted scroll while not confused will produce the confused effect and reading it while confused will produce the regular effect. There is no way to restore a corrupted scroll to normal.

Possibly, an unidentified corrupted scroll will have its label deterministically garbled similar to engravings, e.g. “scroll labeled FO0B|E BLe7Ch”. If identified it would simply show as “corrupted scroll of X”.

Writing a scroll while confused will make the result corrupted. Also, scrolls hit by water damage or produced from polymorphing other scrolls may be corrupted.

Items underneath a cloak of magic resistance in an object pile are immune to being polymorphed by a beam zapped over that pile. (This gives you a way to protect your stash from accidental polymorph.)

The #conduct screen tells how many items you have polymorphed over the course of your game. This is because it will also show up in dumplogs, and can be used to determine how much polypiling was relied on.

Polypiling can’t create matter out of nothing. If an item is trying to polymorph into a heavier item, it must absorb the mass from other objects in the pile, destroying them. Note that if polypiling a stack of same-weight objects (potions, or scrolls), none of the candidate items will be heavier, so this will never trigger.

When a container containing items is polymorphed, it can only turn into other containers (possibly never a bag of holding to prevent abuse). Or else the contents just get spilled out (and possibly polymorphed themselves).

Polymorphing a crocodile corpse makes fireproof water walking boots instead of fireproof low boots.

Polymorph beams stop at the first square on which there’s an object or a monster they affect. In addition, golem formation is made much more aggressive and item-destructive to discourage one-large-stack polypiling.

Reduce the odds that a nonmagical potion polypiles into a magical potion, to disincentivize mass-blanking potions and polypiling them.