aser

An obsolete medieval spelling of asir ("to grasp, seize") found in Old Spanish manuscripts before spelling conventions standardized the -ir verbs. It has no use in modern Spanish outside historical linguistics.

• En textos medievales aparece aser donde hoy escribimos asir. — In medieval texts, aser appears where we write asir today.
• El copista usó aser en vez de la forma moderna. — The copyist used aser instead of the modern form.
• Los filólogos estudian aser como variante histórica de asir. — Philologists study aser as a historical variant of asir.

There isn't a real modern mistake to make with a word this obsolete — the risk is a learner encountering aser in an old text and assuming it's a typo for asar, rather than recognizing it as an archaic form of asir.