I'm using better cities, I think the latest version, and I went into a house near the river in Bravil named "Riverview Mansion." It was really dark and kinda neat, got to the top and found a bunch of corpses, but otherwise not much. Then I made it back to the bottom and the doors on the first level back into Bravil can't be activated and it autosaved when I went into the house.

I also have a floating mushroom in Bravil near the church, something that looks like a flying cow outside the main entrance to Bruma on the path towards the stables, and a land split in Bruma from the church to where the courier building is, but my biggest issue is obviously getting this house to not eat me...

My BOSS load order is:

Oblivion.esm
Cobl Main.esm
Better Cities Resources.esm
Unofficial Oblivion Patch.esp
DLCShiveringIsles.esp
Unofficial Shivering Isles Patch.esp
Better Cities .esp
Cobl Glue.esp
Cobl Si.esp
Cobl Tweaks.esp
TIE.esp
The Ayleid Steps.esp
Knights.esp
The Lost Spires.esp
Better Cities Full.esp
Better Cities - TIE Restricted Area.sp
Better Cities- COBL.esp

I also have Unique Environments + patches, LAME, Window Lighting System, Real Lights, Enhanced Weather, Realistic Physics, Oblivion XP, Horse Fast Travel, and Darnified UI, but pretty sure nothing but maybe that floating cow is influenced by these...

I think I've run into that (the house) before too - it's because the door doesn't have a teleport link back to the outside. In any case, you can just open the console and type "coc bravilfightersguild" (without the quotes); that'll get you out of the house and over to (you guessed it) the FG.

As for the flying cow, you might be able to get the formID (open the console, click on the creature; the first two numbers correspond to the mod's load order position). If it's a levelled list (ff prefix), then all bets are off.