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I tried to run Minecraft, but it went back to the launcher and said this:

[21:35:18] [main/INFO] [LaunchWrapper]: Loading tweak class name cpw.mods.fml.common.launcher.FMLTweaker
[21:35:18] [main/INFO] [LaunchWrapper]: Using primary tweak class name cpw.mods.fml.common.launcher.FMLTweaker
[21:35:18] [main/INFO] [LaunchWrapper]: Calling tweak class cpw.mods.fml.common.launcher.FMLTweaker
[21:35:18] [main/INFO] [FML]: Forge Mod Loader version 7.2.217.1147 for Minecraft 1.7.2 loading
[21:35:18] [main/INFO] [FML]: Java is Java HotSpot(TM) 64-Bit Server VM, version 1.8.0_31, running on Windows 7:amd64:6.1, installed at C:\Program Files\Java\jre1.8.0_31
[21:35:18] [main/INFO] [FML]: Loading tweaker net.minecraftforge.lex.legacyjavafixer.LegacyJavaFixer from legacyjavafixer-1.0.jar
[21:35:18] [main/INFO] [LaunchWrapper]: Loading tweak class name cpw.mods.fml.common.launcher.FMLInjectionAndSortingTweaker
[21:35:18] [main/INFO] [LaunchWrapper]: Loading tweak class name net.minecraftforge.lex.legacyjavafixer.LegacyJavaFixer
[21:35:18] [main/INFO] [FML]: [LegacyJavaFixer] Tweaker: [email protected]03c
[21:35:18] [main/INFO] [FML]: [LegacyJavaFixer] Replacing tweaker [email protected]03c with net.min[email protected]1efed156
[21:35:18] [main/INFO] [FML]: [LegacyJavaFixer] Tweaker: FMLCorePlugin {[]}
[21:35:18] [main/INFO] [FML]: [LegacyJavaFixer] Tweaker: FMLForgePlugin {[]}
[21:35:18] [main/INFO] [FML]: [LegacyJavaFixer] Tweaker: [email protected]03c
[21:35:18] [main/INFO] [FML]: [LegacyJavaFixer] Replacing tweaker [email protected]03c with net.min[email protected]1efed156
[21:35:18] [main/INFO] [LaunchWrapper]: Loading tweak class name cpw.mods.fml.common.launcher.FMLDeobfTweaker
[21:35:18] [main/INFO] [LaunchWrapper]: Calling tweak class net.minecraftforge.lex.legacyjavafixer.LegacyJavaFixer$SortReplacement
[21:35:18] [main/INFO] [FML]: [LegacyJavaFixer] Replacing sort
[21:35:18] [main/INFO] [LaunchWrapper]: Calling tweak class net.minecraftforge.lex.legacyjavafixer.LegacyJavaFixer$SortReplacement
[21:35:18] [main/INFO] [LaunchWrapper]: Calling tweak class net.minecraftforge.lex.legacyjavafixer.LegacyJavaFixer
[21:35:18] [main/INFO] [LaunchWrapper]: Calling tweak class cpw.mods.fml.relauncher.CoreModManager$FMLPluginWrapper
[21:35:21] [main/WARN] [LaunchWrapper]: Problem loading class
java.lang.SecurityException: SHA1 digest error for afn.class
at sun.security.util.ManifestEntryVerifier.verify(Unknown Source) ~[?:1.8.0_31]
at java.util.jar.JarVerifier.processEntry(Unknown Source) ~[?:1.8.0_31]
at java.util.jar.JarVerifier.update(Unknown Source) ~[?:1.8.0_31]
at java.util.jar.JarVerifier$VerifierStream.read(Unknown Source) ~[?:1.8.0_31]
at java.io.FilterInputStream.read(Unknown Source) ~[?:1.8.0_31]
at net.minecraft.launchwrapper.LaunchClassLoader.readFully(LaunchClassLoader.java:298) [launchwrapper-1.9.jar:?]
at net.minecraft.launchwrapper.LaunchClassLoader.getClassBytes(LaunchClassLoader.java:370) [launchwrapper-1.9.jar:?]
at cpw.mods.fml.common.patcher.ClassPatchManager.getPatchedResource(ClassPatchManager.java:59) [forge-1.7.2-10.12.2.1147.jar:?]
at cpw.mods.fml.common.asm.transformers.deobf.FMLDeobfuscatingRemapper.getFieldType(FMLDeobfuscatingRemapper.java:195) [forge-1.7.2-10.12.2.1147.jar:?]
at cpw.mods.fml.common.asm.transformers.deobf.FMLDeobfuscatingRemapper.parseField(FMLDeobfuscatingRemapper.java:172) [forge-1.7.2-10.12.2.1147.jar:?]
at cpw.mods.fml.common.asm.transformers.deobf.FMLDeobfuscatingRemapper.setup(FMLDeobfuscatingRemapper.java:141) [forge-1.7.2-10.12.2.1147.jar:?]
at cpw.mods.fml.common.asm.FMLSanityChecker.injectData(FMLSanityChecker.java:179) [FMLSanityChecker.class:?]
at cpw.mods.fml.relauncher.CoreModManager$FMLPluginWrapper.injectIntoClassLoader(CoreModManager.java:125) [forge-1.7.2-10.12.2.1147.jar:?]
at net.minecraft.launchwrapper.Launch.launch(Launch.java:114) [launchwrapper-1.9.jar:?]
at net.minecraft.launchwrapper.Launch.main(Launch.java:28) [launchwrapper-1.9.jar:?]
[21:35:21] [main/ERROR] [FML]: There is a binary discrepency between the expected input class afn (afn) and the actual class. Checksum on disk is 1, in patch 3bd0f2eb. Things are probably about to go very wrong. Did you put something into the jar file?
[21:35:21] [main/ERROR] [FML]: The game is going to exit, because this is a critical error, and it is very improbable that the modded game will work, please obtain clean jar files.
Java HotSpot(TM) 64-Bit Server VM warning: Using incremental CMS is deprecated and will likely be removed in a future release

It was working fine 2 hours ago, what's the deal here?

Did you manually modify the jar?

No, I've never modified the jar. It was woring today 3 hours ago, but now it's doing this.

So reinstall forge?

Thank you. It works perfectly now