On a multiplayer server, a player was able to grant themselves full invincibility from player damage, including a player in creative.

I don't believe the player to be using any sort of client to create this effect. They claim to be able to do this by "hold the shield and have someone hit it rapidly for 10 seconds". They were able to get full immunity from both creative and survival player damage. Unclear whether they're protected from other damage like mobs. They stayed in this state until they logged off and said they could stop it by right clicking the shield again.