Crash: java.lang.IllegalStateException: Already building!

What the log looks like

java\.lang\.IllegalStateException: Already building!
java\.lang\.IllegalStateException: Already building![\s\S]{1,120}?at net\.minecraft\.client\.renderer\.VertexBuffer\.

How to fix it

1. Identify what you were just looking at or opening when the crash happened (such as a guidebook page, crafting terminal, inventory item, or multiblock hologram), as a modded item or block being rendered there is usually the real culprit.
2. Check `latest.log` right BEFORE the `Already building!` crash report for an earlier suppressed error or rendering exception, which will often name the actual broken mod.
3. Temporarily remove client-side rendering or optimization mods (such as OptiFine, Rubidium/Embeddium, or custom shader mods) to rule out rendering pipeline conflicts.
4. Update the mod whose GUI, manual, or block preview triggered the crash, as well as any mods that have cross-compatibility items displayed on that screen (for example, Chisel, JEI, Applied Energistics 2, Immersive Engineering, or Botania).
5. If opening your inventory or a specific GUI crashes immediately upon loading the world, test with half your mods removed (binary search) in a backup world to isolate which mod's item model is leaving the VertexBuffer open.

Check your own log