The bug

I don't know if this has any effect on the game play but the hitboxes of fireballs and wither skulls (subclasses of net.minecraft.entity.projectile.EntityFireball) are only visible every 3 seconds.

How to reproduce

1. Press F3 + B to show the hitboxes
2. Summon a wither skull

   /summon wither_skull ~ ~ ~ {direction:[0d,0d,0d]}
   

Code analysis

Based on 1.12 decompiled using MCP 9.40 PRE 1

It appears the reasons for this are that constructors EntityFireball.EntityFireball(World, double, double, double, double, double, double) and EntityFireball.EntityFireball(World, EntityLivingBase, double, double, double) set accelerationX, accelerationY and accelerationZ (power NBT tag values) to NaN.
Additionally these values are not sent to the client after they were read from NBT server-side.