solecism

noun3

1. •
   An error or improper usage.
2. •
   An error in the use of language.

   • Eliot and Gertrude Stein had a solemn conversation, mostly about split infinitives and other grammatical solecisms and why Gertrude Stein used them. — 1933
3. •
   A faux pas or breach of etiquette; a transgression against the norms of expected behavior.