When you hold a fire arc or a water arc, it "flickers" downwards every second or two, then goes back to its normal position.

Information obtained via Thread.dumpStack() in EntityArc#setPosition reveal that it has to do with Minecraft's packet system.

Further investigation required.

I experienced a similar problem while creating the Fireball, but interestingly enough, the issue stopped when I changed Vector.getEntityPos to Vector.getEyePos. The issue is now only present with water arc.

Doesn't happen with water arc anymore.
Happens with other entities such as floating block.

Fixed for arcs