If you updated to Minecraft 1.3 and didn't store a backup of your 1.2.5 Jar, you can download
THIS one. Just replace the jar in your bin folder with this one and then just run Minecraft.
Detailed Instructions written by Ako:
How to revert back to an old version of MC:
1. Find the folder called ".minecraft", which is easy to find if you open up MC, click "Texture Packs" and click "Open texture pack folder".
2. Close Minecraft.
3. You will be looking at the "texturepacks" folder, and you need to go up one level to the ".minecraft" folder.
4. In the ".minecraft" folder, click on the "bin" folder.
5. Here you will see all of your old minecraft version files. If you have 'show file name extensions' turned on on your Windows, all of the files will have '.jar' at the end. Locate the one called "Minecraft" or, if you have file name extensions on: "Minecraft.jar".
6. Now, you will back it up by renaming it: call it something useful like "Minecraft-
1.2.2 1.3(backup).jar"
6b. (Optional) You may also backup the Minecraft jar file you are going back to, by copy+paste and renaming it to "Minecraft-
1.11.2.5_COPY" or something.
7. Find the version you need to revert back to (at the time of this post, "Minecraft-
1.11.2.5.jar") and rename it to just "Minecraft" or if you have file name extensions on: "Minecraft.jar".
8. Run Minecraft, and upon logging in, it will ask you to update. Choose "NOT NOW".
And thats it! Enjoy.
~Akomine
http://www.mediafire.com/?3t3tg330n2v531y