Consider a language along the style of Latin, Russian or Finnish, where nouns and verbs belong to conjugations or declensions. Each conjugation or declension provides somewhat different patterns for inflecting your nouns and verbs. Usually in languages such as these, a noun or verb belongs to one in particular, although dialectal, *-lectal and even historical variation may exist. A lexeme might also be somewhat ambiguous as to what particular such class it belongs to.
However, you can also imagine lexemes that in most normal use belongs to a particular class, but have individual exceptions – a certain case or TAM+person or whatever behaving like the lexeme belonged to a different class.
These are fairly plausible exceptions and can be found in many languages. However. A thing that I am unaware of any careful description of are semantically conditioned exceptions. I.e. a word that belongs to a particular class, except for a specific set of phrases including that word - for instance, sausages belong to declination III, but in a particular compound or with a particular attribute that turns it into a specific type of sausage, the lexeme "sausage" is suddenly inflected as though it belonged to a different conjugation. Referring to sausages of that particular type even without the compound or attribute may also trigger the exceptional inflectional pattern.
A couple of things one could do with this:
- restrict this quirk to only some specific case or number - say having the nominative plural behave exceptionally for the specific phrase, but no other case or number; or have it go for all the plurals?
- generalize it as a kind of derivational tool for those lexemes belonging to a particular declension or conjugation
This kind of detail is an easy way of giving one's conlang a kind of 'realistic texture', the kind of 'lived-in' feeling of, say, Finnish, German, Russian, Latin or Georgian.
The reason, btw, why I used the noun "sausage" as an example is that some speakers of Southwest Finnish have that exact distinction, apparently. Makkara has the plural stem makkaroi- in the plural for most sausages, but a few specific types of sausage have makkari- in the plural.