I am rather certain that I once had a Magikarp in my party and when it used its Splash attack on an enemy, it actually did do some damage.
But it never happended again after that and all accounts that I have read so far always state that the Splash move is utterly useless.
Since I did not play very many different editions of Pokémon, it must have happened on one of the Red/Blue/Yellow, Gold/Silver/Crystal editions or the White edition (however, playing White isn’t all that long ago so I don’t think it was there).
Did I dream all this or can anyone confirm my experience?
Splash will never do anything.
You may either be misremembering your Magikarp using Tackle or Flail, both of which are moves that Magikarp can learn that do damage.
You may have also run out of PP for splash (unlikely I know, who’s going to spend 40 moves splashing around to no effect?) then your Magikarp will “Struggle” which will cause damage to both your enemy and yourself.