Im getting these a lot, sometimes even after builds.
I don’t understand. Why is this happening?
I had this fixed for a while cause I added lightmaps but now its back, and I still have the lightmaps on and I have no idea why its doing this! 
I haven’t used UE4 in awhile but it sounds like it is likely that the resolution of the lightmap is too low (I got an email saying that you mentioned me in a comment on here, which is odd since you obviously haven’t).
For large meshes like walls, set your lightmap resolution higher than what you use for smaller objects so that lighting functions correctly. Try setting the resolution to 64 or 128; if that doesn’t fix it then try a higher value like 256, 512 or 1024 (don’t get carried away with high resolutions as it is costly; only use a higher resolution if necessary).
NOTE: Only use power of 2 values for resolution.
@ hey,
Yea I tried a high resolution (256) and even 512 and it didnt seem to work. Any other suggestions?
It mainly just looks like some issue with the lightmap; make sure you’re setting/applying the lightmaps for them correctly (you will have to to find the correct way as it has been awhile for me to suggest off the top of my head).
Other thoughts are potentially the way your lighting is setup in terms of number of bounces, reflection orbs, etc (I’m guessing this is just in relation to the default directional light though?) or your lighting build quality needs to be set higher (low lighting build quality can cause artifacts and incorrect lighting sometimes).
If nothing seems to be working and you are using BSP brushes, you might find creating your own simple mesh with Blender or Maya will fix the issue too (I recall people having issues with the BSP brushes and lighting in the past).
@, May I send a .blend file for you to view?
Here:
Hey, sorry I haven’t replied sooner.
I’m actually a programmer (I have only dabbled in modeling and texturing at a hobbyist level) so I wouldn’t really be able to tell you what you may have done wrong; if you have overlapping UVs, that would definitely cause issues and would be something you’d want to fix up.