Affects Version/s: Minecraft 16w04a, Minecraft 16w05b, Minecraft 16w06a, Minecraft 16w07a
Fix Version/s: Minecraft 1.9 Pre-Release 4
In the latest snapshot the mercy period was removed, meaning that damage is done as fast as people can click at another person rather than limited to a certain speed.
This makes no sense given the developer's stated aims to remove spam clicking.
I recorded a clip of the bizarrely fast attack speed
Edit: I've learned that this applies to other forms of damage as well.
Steps to reproduce:
1: log into a multiplayer server in 16w04a
2: go into survival mode and find some lava
3: jump in.
1: log into a multiplayer server and spam click at another player.
2: observe nearly every hit land, faster than possible in 1.8