This only applies to brushes, not overlays. It could be that you've put the overlay over 2 brushes and Hammer doesn't know which to render it onyour textures are more or less doubled, causing the flickering, its normal, but two textures on the same level will fight for view ability.
Z-fighting only occurs on brushes. Hammer does mess with overlays because they only render on single brushes. I'll put this here again so you can read it:It's really two textures not seamlessly fitting, overlays or not, the same texture on two brushes can glitch. So it's hammer derping in all senses, and last I checked: hammer didn't mess with overlays as they would only render on single brushes, and divisions would break them, or be cut at the seam line.
It could be that you've put the overlay over 2 brushes and Hammer doesn't know which to render it on
The hazard tape texture has an overlay version and a non-overlay version. Make sure you're using the one that has "overlay" on it. (Or "decal"... I forget which.)I've been getting this problem with the caution tape texture. I'm pretty sure that's meant to be used as an overlay...
This shouldn't be an issue either, though, since what Source does with overlays (and possibly also decals? ...no, I think decals are different and that's why they're locked to a certain scale and orientation) is to float it a barely perceptible distance away from the brush it's on; you'll notice this if you try to stretch one across a convex corner and apply it to both faces, because there'll be a tiny gap between the two that you can never get rid of.This only applies to brushes, not overlays. It could be that you've put the overlay over 2 brushes and Hammer doesn't know which to render it on
I think I have had this issue before, although it might have been when trying to put an overlay on the border between 2 displacementsThis shouldn't be an issue either, though