Learn English – Fish Irregular Plural Forms

Etymonline has this to say:

-ics
in the names of sciences or disciplines (acoustics, aerobics, economics, etc.) it represents a 16c. revival of the classical custom of using the neuter plural of adjectives with -ikos (see -ic) to mean "matters relevant to" and also as the titles of treatises about them. Subject matters that acquired their names in English before c.1500, however, tend to remain in singular (e.g. arithmetic, logic).

So yes, at some point in history, there were such things as physic (meaning "natural science"), mathematic (meaning "mathematical science"), etc. that were later turned into plural forms but kept being treated as singular.

Edit: having looked in a few more places, it appears that in contemporary English, it still makes some sense to have both the suffix -ic and its plural form -ics. According to the Collins English Dictionary, the former has kind of specialized in forming adjectives, while the latter is happily forming nouns:

-ic
suffix forming adjectives

1. of, relating to, or resembling: allergic, Germanic, periodic. See also -ical.
   [...]

[from Latin -icus or Greek -ikos; -ic also occurs in nouns that represent a substantive use of adjectives (magic) and in nouns borrowed directly from Latin or Greek (critic, music)]

[...]

-ics
suffix forming nouns (functioning as singular)

1. indicating a science, art, or matters relating to a particular subject: aeronautics, politics
2. indicating certain activities or practices: acrobatics

[plural of -ic, representing Latin -ica, from Greek -ika, as in mathēmatika mathematics]

The key here is that they are not just two unrelated suffixes. Much rather, one is etymologically a plural form of the other. As the American Heritage Dictionary succinctly puts it, -ics is "-ic + -s".

There are a few issues that seem to be in play.

In most cases, a reference to plurals in rules also includes the singular. And vice versa.

In the event of errors, accommodation will be made.

While the plural is used, most readers would believe that this applied to a single error as well as multiple errors. Similarly

In the event of error, accommodation will be made.

It means the same thing.

Sometimes, to make clear that both singular and plural circumstances apply, parenthetical plurals are used.

In the event of error(s), accommodation will be made.

This makes it explicit that both singular and plural problems will be adjusted for.

I am not aware of a term that names this characteristic (or conundrum).

As you have suggested, this is a problem often faced in legal drafting (which you wish to avoid). To ensure clarity, many contracts say something that means "plural words can cover singular events and singular words can cover plural events."

To convey this simply you could say

Amendment shall be permitted only in cases of: 1. deletion of one or more claims 2. correction of one or more errors.

Your example also raises another issue. It lists two conditions:

1. deletion of claims 2. correction of error

There is some ambiguity as to whether these conditions are conjunctive or disjunctive as written. Does it mean

when there is both 1. deletion of one or more claims AND 2. correction of one or more errors

or does it mean

when there is either 1. deletion of one or more claims OR 2. correction of one or more errors, BUT NOT BOTH

or does it mean

when there is either 1. deletion of one or more claims OR 2. correction of one or more errors, OR BOTH

You need to clarify whether you mean

• 1. AND 2.
• 1. BUT NOT 2.
• 1. AND/OR 2.