I am having this exact same "Missing class member: public B c, class: agl, mod: Forge_1.4.2.zip" problem.
I am also using the Magic Launcher and Magic Launcher records one error with the Forge mod when it is added to the configuration. See the Magic Launcher log below.
Note:
I am using the most recent build of Forge (universal-6.0.1.341 for Minecraft 1.4.2).
I have tried the latest Forge build (minecraftforge-universal-6.0.1.345 for Minecraft 1.4.2) but the same error occurs.
I am running Magic Launcher 1.0.0.
Forge was added correctly to the configuration (i.e. first, at the top of the list, before any other mods).
When other mods were applied to the configuration, no errors were reported by Magic Launcher.
Upon Launching Minecraft from Magic Launcher using the Forge-modded setup, FML appeared to setup the mods without a hitch.
However, upon selecting a world to play, the game then crashes to a black screen.
In the log files you will notice that I have renamed my mod zip files, Forge_1.4.2.zip relates to the latest build of Forge for Minecraft version 1.4.2. In the log file below, the specific Forge version in question is 6.0.1.341.
Below is the ForgeModLoader-client-0 log.
I'd love to be able to continue using Magic Launcher but I could be convinced otherwise. However, I would like to think the Forgers would be interested in fixing such bugs anyway so I hope the details I have provided can help point out the problem.
Thanks in advance,
Abe