Searched the whole world wide web without seeing this problem (close to it).
I use Mod Organizer for my Fallout 3 modding, and STEP has been my grail, mostly. I've modded for awhile now, so knowing what is hazardous and not, I follow STEP for MO since it's a new tool. After getting a fairly modded FO3 and running stable for hours of play, I head into the sewer entrance near Super Duper Mart, and go into county mainline sewers. Save inside, then try to load and it starts crashing. Others have had issues with this, but I don't think this is my problem.
I have quite a few exe's running through MO to get everything working and stable, FO3, then FOSE, with FOMM for the UI mods, then Fallout 3 Fullscreen mod, which btw is a lifesaver practically eliminating CTDs. Trying to start again the FO3Fullscreen says, "error: #maxmem full, shutting thread" and after lots of research I find my falloutprefs.ini for the MO profile has grown. A lot. Where it was 3 kb it's now 80 mb. Pull it up in Notepad ++ and what happened is the entire file has copied itself into itself, probably thousands of times, so much that I thought after hitting CTRL + END it locked up, just took awhile to get to line 3 million plus. After deleting the excess the game was perfectly playable, but it starts copying and growing like before. Thought I could make it read only, but MO won't let you run it with the RO flag. Thought it might have to do with the multiple exe's, but I've not found this problem, yet. Any ideas would be appreciated. Running 85 plugins, so let me know if you want the list, but mostly the major, popular mods, not much else.