Error: An exception occured when compiling Mod Package to JAR

Many things can cause this error to occur. Unfortunately, there is no one method to solve this problem for everyone,  so it’s recommended you upload the contents of your error.log output either here in the comments section or in the Minecraft forum thread. To find error.log, use this guide.

But there is a few things you can try in the meantime:

For those who get the additional “An error occured extracting [file]”

Minecrafter had a problem extracting this particular file. The solution is to remove the mod which contains the file, and repackage it to a ZIP. This errors usually only occurs on RAR archives, but may show up on 7Z archives.

Close any programs using minecraft.jar

Sometimes you may have archive utilities with minecraft.jar open, or even Minecraft itself. Make sure you close these programs before launching your mod package. A trick to see if minecraft.jar is locked is by renaming it. If you can’t rename minecraft.jar, then there is a program using it.

Restart Minecrafter

If you’ve just created a mod package and attempted to launch it, some circumstances may cause this error. Try restarting Minecrafter and launching the mod package again.