no_entry overlay is missing?

That's most likely a case of the good ol' "missing textures because Hammer was open during a TF2 update or something similar to that".

Go to Steam->right-click Team Fortress 2 in the game library on the left->Properties->Local Files->Verify integrity of game cache. Make sure Hammer isn't running, as well as GCFScape or stuff like that.

I also experience this issue. I believe it is because the material is using a proxy to adjust its opacity based on the player's distance. Hammer doesn't work well with texture transforms in VMTs, as evidenced by the strange rotation phenomenon if you happen to use the security glass texture anywhere in your map.

The reason you are able to see a preview of the texture at all in the materials browser is because of the special preview image that A Boojum Snark included in his mapping resource pack. This is one of the many improvements to the material browser that assist the mapper in finding materials that are hidden or obscured in the default TF2 Hammer setup.

If you would like to see the texture in Hammer's 3D viewport, to make it easier to adjust its size and position, change the viewport's mode to 3D Textured. This can be done by clicking the little 'camera' button in its top left. This also takes care of the security glass issue and allows you to see envmap reflections in reflective surfaces such as window glass.

That particular texture just doesn't render properly in the editor when using Shaded Textured view - similar to how other overlay textures flicker in this camera mode when used in an info_overlay. Switching to the Textured view will fix these visual issues in Hammer, but rest assured that it will render correctly in-game either way.