"Forge mod loader has found a problem with your installation"
Then some stuff about Dummy Container, and yadda yadda.
You're gonna ask for ForgeModLoader-client-0.log, just like the game did, however I couldn't find it among my game files.
I've played FTB in the past and it seems to have ended up in with those files somehow. However, what's printed in the log calls back to a game played almost 2 months ago, it's not even the right version, and calls to mods I don't even use. And to top off this expostional onslaught: it's the only copy I could find on my entire hard drive.
So yeah, based on prior knowledge I deleted that, re-ran minecraft, and now it straight up doesn't exist.