I bet I sound like a noob who's too lazy to update my mod.
Here's what's happening: I was updating my mods for 1.5.2, and one of them - the Ear Slimes Mod(http://www.planetminecraft.com/mod/ear-slimes-mod/) - showed no errors when Eclipse had built the workspace. So I tested it, and it worked just fine. So I tried running it with the official Minecraft launcher (compiled, obviously), and Minecraft crashed without generating a crash report. I made sure it wasn't another mod's fault, and that MCP had correctly updated. Minecraft did spit out a file called "ForgeModLoader-client-2.txt". That file:
The part that actually seems important/useful:
If anyone knows what the issue is and/or how to fix it, please reply! This is one of my more popular mods, and I hate telling people it might not be update-able!