Tbh it seems entirely plausible to me that a messenger being unable to deliver an important message could have an outsized effect on that outcome of a battle. What if they're letting their side know about a surprise attack?

Seems also plausible that risks might apply to the messenger that wouldn’t apply to the troops in garrison—that is, the thousands of other horseshoe nails in inventory could have gone unmissed or doomed a less important horse.