This is really hard to put into words, so I’ve prepared some screenshots and a repro file. Make sure you’re viewing this on a screen with adequate brightness and that doesn’t have other light (e.g. sunlight) glaring onto it.
Examine the following screenshot:
The first thing you’ll notice is that the helmet in the character has this weird black glow to it. For some reason it’s not showing up on that upper spot on the helmet. If you look towards the lower part of the helmet’s visor, you can see a horizontal line – that’s the CornerWedgePart behind the helmet. Because this is a gif, its low quality makes it hard to see, but if you look close enough you can see the edges of the CornerWedgePart as I move my head:
This does not happen when the helmet does not have a decal applied. It also applies to MeshParts in the body when a decal is applied to them. Here is a repro file that you can use to reproduce what I did in the gif and image:
Repro.rbxl (34.5 KB)