advertir

Advertir is the historical form of avertir (to warn) — the same verb before French phonetic evolution dropped the 'd'. It survives only in etymology and in a handful of archaic or dialectal texts; modern French uses avertir exclusively.

• L'ancien texte disait : "je vous advertis de ce péril." — The old text read: "I warn you of this peril."
• Ce mot n'apparaît plus que dans les écrits anciens. — This word now appears only in old writings.
• Les linguistes citent advertir comme ancêtre d'avertir. — Linguists cite advertir as the ancestor of avertir.

There's no real mistake to make here in modern usage — the only pitfall is using advertir at all in contemporary French, where avertir is the sole surviving, correct form.

Often confused with avertir.