Skybox textures can have a seam when using an inverted cube for a sky.
Attached is an example 4.24.2 project.
Note: One of the attached seam screenshots is hard to see unless you expand it.
Found in 4.24.2 CL#11100242
Reproduced in 4.23.1 CL#9631420, 4.26 Main CL#11411855
1. Import a 4096x2048 24 bit .tga file (SkyTex).
2. Import a cube that has inverted normals.
3. Create a material(SkyMat) from SkyTex.
a. Set SkyMat to Unlit.
b. Recreate the material in the attached image.
4. Place the cube in a new scene and scale it to 1000.
5. Delete the default skysphere actor.
6. Apply SkyMat to the cube.
Result: There is a seem in the material.
Expected: There isn't a seem in the material.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-88768 in the post.