Riven

considering it is super easy to mess up, and the fact she is doing a freaken summer sault, both mechanic wise, and realism wise it makes perfect sense for it to be a wall jump

It was originally discovered by players as a unintended side effect of some map upgrades that were made, but it was very hard to do. So many people learned to do it that riot addressed the issue and decided that the third q wall jump already had a lot of restrictions on it and fit her design as a mobile light fighter, so they left it in and later made it easier to do by supporting it as an intended interaction instead of a bug.