If you use /kill on a player, they are unable to die by any means, including mobs, fall damage, hunger, etc.
Restarting the world does not fix the issue, meaning you likely have to load a backup to continue playing
Seems to be caused by [13:43:44] [Server thread/WARN] [firstaid]: New current health is not a number, setting it to 0!
latest.log

NBTEdit on the player's .dat file will fix this without resorting to backups. This issue is in tips and book. There's an issue with the firstaid mod, obviously.

Unfortunately, this issue has no fix that we're aware of. The workarounds are to not use /kill, and NBT edit the player to fix if it's already occurred.