Entities like thrown snowballs and fishing rod's bobber are invisible on SMP. You can still hit mobs with snowballs and reel them in with a fishing rod, it's just that they're not visible.
This bug exist on Forge build #94, and checking newer builds' changelogs, it wasn't noticed yet.
I'm using:
Client:
ModLoader 1.2.5
Forge build #94
IndustrialCraft2 v1.95b
Server:
Forge build #94
IndustrialCraft2 v1.95b
IndustrialCraft2 mod is surely not the reason behind this, bug exists even when launched without it.
I'm attaching ModLoader.txt, as requested.
ModLoader.txt
