There is a common bug that many people like to use. That is, the jump and then cast an ability. The ability won't cast, but it will play the initial sounds and animations.

This bug isn't particularly problematic, unless it also "consumes" mana client side. Then the player must reset the state somehow (casting an ability for real, dying, or restarting).

Here is an interesting demonstration (The first half has been shown many times, I understand, but the second half shows that the bug does not exist in jungle practice).

More importantly, there is a casting bug that impacts game play heavily! I demonstrate it below. Flicking the targeter onto a wall wall after it's been cast will cancel the ability mid animation... This has happened to me during play multiple times on at least Ullr and Fafnir. It may be connected to the bug above.