Pushing a glass block into the head of a mob will cause it to take suffocation damage. If a player is suffocated, the screen goes black as if the glass is an opaque block. Normally, one would expect glass to act as the transparent block that it is and not suffocate mobs that collide with it. This has become an issue as of only recently, so there is little chance that it is intended.
The image below illustrates this issue very clearly: a villager is taking damage because a glass block has been pushed into its head.