Players are invincible for 20 ticks after they connect

[RhysB]

Minecraft makes the player invincible to damage for the first 20 ticks of connection. This allows players to rejoin before they are about to dye (fall damage and etc.) to prevent damage. It might be best to allow users to set a custom invincibility tick time in the Poseidon config. We will need to do testing with these changes to find any bugs as this might exist for a reason.

The invincibility timer is defined in EntityLiving.java

[I-Tom-I]

This exists in modern Minecraft too so it's an intended feature not a bug