While testing Sync on our SMP environment, we had a player which ended up being invincible: lava, pvp & fall damages had no effect, /kill would kill him but remain invincible after respawning, he wasn't in god mode nor creative. Disconnecting didn't help either. Making another sync was ok but the original shell was still invicible.
Checking his player data, I've found an "invulnerable" set to 1. Putting it back to 0 solved the issue.
Technically, it died twice in a row, the second time while respawning in his shell.

According to the player, this started as is:

• create a sync
• die => respawning in shell but die a few seconds later (first bug)
• respawn => invincible (second bug)

I managed to get a similar invincible effect when I logged out in the middle of switching between shells while my viewpoint was still up in the air, but I can't reproduce it 100% of the time. When I was trying to reproduce it I also had occasions where I logged in inside of the shell constructor that I was transferring to, except there was still an empty shell inside (essentially giving myself a free shell).

I had another instance of the invincibility bug after crashing while I was walking around as a shell. The crash itself doesn't seem to be related to this mod at all but I noticed that I wasn't able to /kill myself or be hurt by anything anymore upon logging in.

Possibly fixed with 1b419d0 and 2770aa4. Seems to be similar to #108

Seems to be fixed, cannot replicate. Tested both on server and client.