Problem
Many items in the game have no plural form that can be used when applicable. This affects wishing (you only get one) and grammar ("heap of 3 cheap cloak").

The following is a list of items with no plural, in "unIDed appearance/IDed name" format wherever applicable.
figurine/figurine of wondrous power
brass bracers
silver bracers/bracers of defense
copper bracers/bracers of protection
steel bracers/bracers of regeneration
iron bracers/bracers of resistance
bronze bracers/bracers of speed
bone bracers/bracers of toughness
manual of bridge building
cheap cloak/?
ugly clothes
fancy clothes
light furs
thick furs
thieves picks
scribbled scroll/scroll of warning
scroll labeled "Fox Nawagu"/strange message

There are the plural forms I'd suggest to fix the problem.
figurines/figurines of wondrous power
pairs of silver bracers/pairs of bracers of defense
pairs of copper bracers/pairs of bracers of protection
pairs of steel bracers/pairs of bracers of regeneration
pairs of iron bracers/pairs of bracers of resistance
pairs of bronze bracers/pairs of bracers of speed
pairs of bone bracers/pairs of bracers of toughness
manuals of bridge building
cheap cloaks/?
sets of ugly clothes
sets of fancy clothes
sets of light furs
sets of thick furs
sets of thieves picks
scribbled scrolls/scrolls of warning
scrolls labeled "Fox Nawagu"/strange messages

How to reproduce it
Obtain more than one item in the list, or try to wish for the same amount.

Notes
The manual of bridge building, scribbled scroll and strange message are typically one-of-a-kind, but more can be wished for. This leads to odd grammar ("heap of 2 uncursed strange message").

Manuals of bridge building stack only if they have the same number of charges (a property revealed by Adombot), which can lead to "heap of 2 uncursed manual of bridge building".

Cheap cloaks only lack a plural on their unidentified form. All their possible identified names have their corresponding plural.