So...this happened in servers before it happened on worlds, servers are fine now. Should I go to Windows 10 beta or not?
Oh, and to reproduce:
1. Get a website whilst playing Minecraft
2. After building something, leave Minecraft then take website off
3. Turn off PC and wait.
4. After a long while (1 day, 10 hours, etc.) turn it back on.
5. Get on Minecraft and get on the world that you built on.
6. It should have a crash report.
p.s only for the first time
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