Admiral William Penn
In 1654, Admiral Willam Penn and General Robert Venables led a group of men into the reeds off of Rio Hayna. The men were in canoes that had disembarked from large troop carriers parked in the shallow waters off the coast of Jamaica. This operation was to take Jamaica from the Spaniards in a surprise attack. Asd they approached the reeds, the men could clearly hear a loud rustling sound, growing in volume and strangeness with each passing moment.
The relentless, unexplained din gave them the impression that a very large group of men was about to burst through the reeds and attack them. The noise increased and until the unnerved men in the front of the line peeled off and made haste to the boats. The rest followed them into the canoes and they all returned to the ship.
Upon further investigation, the source of the sound was a large group of crabs migrating through the reeds.
Source: Wiki, Military Anecdotes by Geoffrey Regan, Carlton Books, 1992.