1. Was playing in a Hardcore world
2. Died (first from fire, recreated event by drowning in a different world and got the same results)
3. Clicked "Spectate World"

I then ran into the following

• Blocks/entities wouldn't load
• I couldn't move
• Items/exp orbs were stationary
• Nearby mobs spinning rapidly (only the first time. second time most were idle)

*Edit: Optimizing the world fixes the issue