At high velocities (easy to observe at anything higher than 6 meters/tick), projectiles look like they go in another direction than you're aiming. How much they deviate depend on your angle: In cardinal directions (north, west, et c.), they do not seem to deviate at all, but if you move slightly toward an intercardinal direction (NW, SE, et c.), they seem to deviate further in that intercardinal direction. If you aim straight in an intercardinal direction, they go straight in that direction.

Yet the arrow entity does not move in that direction; the effect is purely client-side, it seems.

If the arrow is critical, it also seems as if the particle trail originates to the right or left of you, depending on which direction the arrow is going relative to your aim.

At extremely high velocities (tested at 100m/t), they deviate so heavily that they hit the ground and you can even hear an arrow collide with something - presumably the ground.

I have tested this with arrows and snowballs.