I am not sure myself whats going on.
I am sure Tim, Erik or so can go more in depth about it.
who knows, maybe there is so much bouncing going on between those smushed together polygons that the engine goes “Screw you luos, im going to get a beer” 
@Roball and Luos:
At the moment Eric and I are putting together the Lighting Troubleshooting guide and Tips: A new, community-hosted Unreal Engine Wiki - Announcements - Epic Developer Community Forums
At the moment it is coming along. The two big ones that we want to workout and get up soon are Lightmass Troubleshooting and Shadow Seams. I started actively working on this one this past week, but it’s something that I only get time to do when I can make the time to do so. At the moment I’d say I’m about a quarter of the way done with it. There is a lot of factors that can go into what is causing the shadowing issues people run into and just as Luos showed in his video I have to set up examples and screenshots for all of these. I don’t want to miss something in that process that someone would try and then question if that would work. I’m trying to be thorough about it because these questions come up very often and if there is a good example provided it’s much easier to provide that for someone than going back and forth for days. Unfortunately, we’re not all robots that can go on forever. We all need a little sleep sometimes. 
@Leam:
My suggestion here would be to take a cue from the architectural assets included with the starter content. These all are scaled to fit together and they exceed the 0,1 UV space for the texture UV. The biggest thing here is make any textures you decided to use work at that scale. If someone wants to apply a texture that doesn’t fit that scale it would be time to learn about the Texture Coordinate node in the Material Editor to scale it to fit. 
If anyone wants to track the progress of the Lighting Troubleshooting and Tips guide they can do so here: [TUTORIALS] Photons Be Free: Mini-tutorials and other curiosities - Community & Industry Discussion - Epic Developer Community Forums
Grabbing a beer sometimes can help any frustration that would be caused otherwise.
Most def!
offtopic:
Not sure who I will meet at GDC but im sure to pass on some hugs!
flight in 6 hours.
Our support team lead, Stephen Ellis, will be present along with some other kick[butt] people from Epic! 
The issues you are experiencing are due to the UV shell edges not being snapped to the pixel grid and the second UV channel (which is used for the lightmap unless otherwise specified in UE4) doesn’t have the UV shells separated and spaced appropriately, resulting in shadow bleeding. Faces that are adjacent on the mesh should be separated/split if the normals differ significantly.
Left: Diffuse UV, Right: Lightmap UV.
I only skimmed through your video, but your lightmap UV needs adjustments. As mentioned above, faces that are adjacent on the mesh should be separated/split if the normals differ significantly. In addition, spacing the UV shells a mere 1 pixel at 512x512 is setting yourself up for failure, as there isn’t much room for error and the downside is that you can’t use a lightmap resolution of less than 512x512, as you’ll get shadow bleeding. For simple meshes, spacing the UV shells by 1-2 pixels at 32x32 (or 64x64) is easily achievable, more flexible, and will likely produce better results. Also, in your example, the side faces don’t need to be deleted, but that can help in cornered sections (as Tim has demonstrated). Using the lightmap UV I attached above, here are the results of placing 5 meshes adjacent to each other using the default lighting and a lightmap resolution of 64x64.
If you’d like to try it for yourself, the following URL contains a ZIP file that includes the Maya (MA) file, FBX file, and a screenshot of the UE4 FBX import settings.
http://www.filedropper.com/smwall
I hope this helps!
Edit: I’ve also uploaded the following video.
