Explanation

1.20 introduced decorated pots, which can (apparently) be made using sherds. By defining a sherd with an invalid namespace in the item's NBT, the game will instantly crash for anybody who attempts to render the pot, including yourself. This can be very easily exploited as it crashes anyone who renders a player with the item in their hand.

Steps to reproduce

1. Load a world that you don't care about
2. Use this command: 

   /give @p decorated_pot{BlockEntityTag:{sherds:["please:fix_this!"]}}