Here's a pic of whats going on:

http://melrosebison.com/lightbleed.jpg

Why is light coming in from under this wall? I've replaced the wall, made sure everything is lined up perfectly. And yet I cannot stop the light coming in from under the wall.

???

is ur light a primary light?
are there any lightmap errors while compiling?

This happens to me also, its very annoying.

The light coming in under the wall is from outside, the world.

There isn't a defined way to fix this error, but there is some ways to attempt to fix it.

1. Redo the brush and make sure its connected or goes through the floor.

2. Put a caulk brush on the seam, much like caulking a piece of trim so air doesn't get through.

Tried all of the above. Still no luck.

I used to get this a fair bit, bit of a pain. Various thing can help for different situation,

Try widening or moving the brush a little.
Try using the shadowcast texture behind the wall
Make sure you have lightgrid and refection probes in the area.
Move any light entities so the inner radius isnt touching the wall
Editing the lightgrid with SHIFT+L can help.
A few times i found placing an entity near the wall can fixit (not sure why)

See if any of this can help. Good luck.

Tried widening-moving brush - no good.
Tried shadow texture behind wall - no good.

I have a reflection probe in the area - my lightgrid surrounds the entire map. I created another lightgrid volume and filled the room with it - same thing - no good.

I've removed all lights in the room - still no good.

What am I looking at when I do shift+L? Won't that change the entire map?. Not sure how to use the function..

I had this happen with a red light from a basement. To fix it I made the wall and the floor structural.

The only way i can think of a fix that might work is to do the wall/s in 2 pieces and make the outside wall a little longer and/or 45 degree the wall where it meets the floor?
R1