When you boot up Minecraft and join a realm that is meant for 1.16.4, you will get into the realm, but the time system will be weird.
Constantly switching between the realm's time and the client's time.
After you quit and go to a singleplayer world, if you're in windowed mode and tab out before the world loads, the world will still load and show like you're holding the realm's items. (They disappear if you unpause the game.) 
If you unpause and pause the game and wait for a little while, you will start taking damage.
It will then say you were slain by a zombie with a fake death screen.
I found this very hard to recreate.