unintentionally

One of the two men, whose mysterious meeting in the Colosseum he had so unintentionally witnessed, was an entire stranger to him, but not so the other; and though Franz had been unable to distinguish his features, from his being either wrapped in his mantle or obscured by the shadow, the tones of his voice had made too powerful an impression on him the first time he had heard them for him ever again to forget them, hear them when or where he might.

“Do you think the same hand which unintentionally struck Barrois has now attacked Valentine?”