Happened to me as well. I was installing the latest Forge build into a virgin 1.5.2 jar. (Yes, I deleted the original META-INF folder, this isn't my first modding rodeo.)
Installed Forge and booted Minecraft. Was working for a bit then suddenly...
My ForgeModLoader-client-0: http://pastebin.com/45SxYKxS
Please let me know what I can do to resolve this. Thanks!