1900 (pre): The Kelpie and the Water Wheel
In a 1900 book by John Gregorson Campbell, the author writes that a friend of his from the Shetland Islands, Scotland, claimed that the well-known Scottish water monsters called 'Kelpie' were a sort of mischeivious prankster. This friend then told Campbell of hearing a man declare that his mill-wheel stopped turning one night for a whole half of an hour... and the man was too frightened to go out and see what could prevent a water-wheel from turning. This miller "not only said but maintained that it was a Kelpie or something of that kind that did it."
...which means "Friend Of A Friend," which is the worse chain to get a supposedly true story from! The story above is mentioned in the book by Campbell as an example supporting his position that in Scottish legends 'ech-uisge' ["water-horses"] and Kelpie are two different monsters, with Kelpies being non-vicious and more playful...which is in direct disagreement with every other author on the topic before him. Unfortunately for Campbell Shetland is an island separated from the mainland of Scotland, so a difference in beliefs out there does not override what all legends printed previously indicate: the names "water-horse" and Kelpie do both stand for the same monster.
It's also hard to even say that the miller in the story above -- if he existed and the event happened -- even felt it was a Kelpie that stopped his water-wheel. The somewhat non-commmital quote above only seems to state he felt it was a spirit of some sort causing the problem, with a Kelpie simply being his first guess/suggestion.
So, not reliable as an account of the supernatural... but still note-worthy as a varient legend about Kelpies from Shetland.