Introduction: Worlds created before 1.13 store the note information of a note block as NBT, that means (in Minecraft up to 1.12.2) any note block could be CTRL/CMD-picked and retain the note information when placed. Pre-1.13 worlds opened in 1.13 up to 1.15.2 will load with the inventory cleared if the player has a note block with attached NBT. Starting from 1.16, loading those worlds causes the game to crash.
Steps to reproduce:
1. Open a pre-1.13 Minecraft installation. Create a world
2. Place a noteblock
3. CTRL/CMD-Pick the note block. Keep it in the inventory
4. Save the world. Close Minecraft
5. Open a 1.16 to 1.16.4 Minecraft installation
6. Load the previous world
Observed result:
Minecraft crashes with the following error:java.lang.IllegalArgumentException: Unknown type minecraft:noteblock in akn$$Lambda$476/1476061457
Expected result:
The world loads with the player inventory cleared
Additional notes:
Opening a pre-1.13 world in 1.13 up to 1.16.4, which has a chest containing a note block with note NBT, causes the game to regenerate the chunk in which the chest was placed. Considering the added warning before loading pre-1.13 worlds, and the fact the game somewhat gracefully fails while still properly rendering the other chunks, the chunk-corruption issue doesn't seem to be of higher priority.
Can confirm in 1.16.5 and 21w03a.