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I used NBTExplorer to check my seed. I made no changes that I am aware of, however my save file now crashes when opening.

---- Minecraft Crash Report ----
// Oh - I know what I did wrong!

Time: 11/22/13 12:09 PM
Description: Rendering screen

java.lang.RuntimeException: Failed to check session lock, aborting
    at ajt.h(SourceFile:45)
    at ajt.<init>(SourceFile:31)
    at ajp.<init>(SourceFile:16)
    at ajq.a(SourceFile:83)
    at net.minecraft.client.Minecraft.a(SourceFile:1363)
    at axu.e(SourceFile:166)
    at axv.a(SourceFile:218)
    at axt.a(SourceFile:175)
    at axu.a(SourceFile:191)
    at bfq.b(SourceFile:768)
    at net.minecraft.client.Minecraft.K(SourceFile:597)
    at net.minecraft.client.Minecraft.run(SourceFile:526)
    at java.lang.Thread.run(Unknown Source)


A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Stacktrace:
    at ajt.h(SourceFile:45)
    at ajt.<init>(SourceFile:31)
    at ajp.<init>(SourceFile:16)
    at ajq.a(SourceFile:83)
    at net.minecraft.client.Minecraft.a(SourceFile:1363)
    at axu.e(SourceFile:166)
    at axv.a(SourceFile:218)
    at axt.a(SourceFile:175)
    at axu.a(SourceFile:191)

-- Screen render details --
Details:
    Screen name: bkf
    Mouse location: Scaled: (152, 116). Absolute: (305, 247)
    Screen size: Scaled: (427, 240). Absolute: (854, 480). Scale factor of 2
Stacktrace:
    at bfq.b(SourceFile:768)
    at net.minecraft.client.Minecraft.K(SourceFile:597)
    at net.minecraft.client.Minecraft.run(SourceFile:526)
    at java.lang.Thread.run(Unknown Source)

-- System Details --
Details:
    Minecraft Version: 1.5.2
    Operating System: Windows 7 (x86) version 6.1
    Java Version: 1.7.0_45, Oracle Corporation
    Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Oracle Corporation
    Memory: 474041104 bytes (452 MB) / 519110656 bytes (495 MB) up to 1037959168 bytes (989 MB)
    JVM Flags: 2 total; -Xms512m -Xmx1024m
    AABB Pool Size: 0 (0 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
    Suspicious classes: No suspicious classes found.
    IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
    LWJGL: 2.4.2
    OpenGL: GeForce 8300/PCI/SSE2/3DNOW! GL version 3.1.0, NVIDIA Corporation
    Is Modded: Probably not. Jar signature remains and client brand is untouched.
    Type: Client (map_client.txt)
    Texture Pack: Default
    Profiler Position: N/A (disabled)
    Vec3 Pool Size: ~~ERROR~~ NullPointerException: null
1

According to this beastnode article the problem is easy to fix.

Open the folder where your save is located (minecraft folder/saves/save name) and delete the session.lock file.

  • Ok, that worked, but now my world no longer has any of my structures? – Adam Nov 22 '13 at 17:51
  • As in stuff that you built? If that's the case, it may have regenerated the terrain. – MBraedley Nov 22 '13 at 18:25
  • I think your NBTExplorer use is just coincidental. It sounds like Minecraft shut down hard. That could leave a session.lock lying around, and possibly corrupt one of your regions mid-write, which would cause that part of your map to regenerate. – Justin Aquadro Nov 24 '13 at 18:12

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