I was recently reading up on the stealth and cover mechanics, and even though I was fairly certain about what is and what is not possible, I found out that one edge case isn’t particularly well-documented.
The rules, to be exact the Stealth rules correction from Player’s Handbook 2, state:
Becoming Hidden: You can make a Stealth check against an enemy only if you have superior cover or total concealment against the enemy or if you’re outside the enemy’s line of sight. Outside combat, the DM can allow you to make a Stealth check against a distracted enemy, even if you don’t have superior cover or total concealment and aren’t outside the enemy’s line of sight. The distracted enemy might be focused on something in a different direction, allowing you to sneak up.
So, what it especially says is that “superior cover” works as a basis to get hidden behind. According to the Dungeon Master’s Guide on determining cover for ranged attacks:
Choose a Corner: The attacker chooses one corner of a square he occupies, and draws imaginary lines from that corner to every corner of any one square the defender occupies. If none of those lines are blocked by a solid object or an enemy creature, the attacker has a clear shot. The defender doesn’t have cover. (A line that runs parallel right along a wall isn’t blocked.)
Superior Cover: The defender has superior cover if no matter which corner in your space you choose and no matter which square of the target’s space you choose, three or four lines are blocked. If four lines are blocked from every corner, you can’t target the defender.
So, in theory, if you’d have a situation where you’d have superior cover from an enemy, e.g.
you’d be able to stealth yourself and gain combat advantage.
The only thing that really denies this possibility are, again, the Stealth updates from Player’s Handbook 2, this time the “Remaining Hidden” section [emphasis mine]:
Keep Out of Sight: If you no longer have any cover or concealment against an enemy, you don’t remain hidden from that enemy. You don’t need superior cover, total concealment, or to stay outside line of sight, but you do need some degree of cover or concealment to remain hidden. You can’t use another creature as cover to remain hidden.
Many thanks to @Milambus for looking up that passage. [And making me feel stupid for not having found it myself, by the way.]
And that’s the only problem. So, you could gain stealth moving behind enemies, but immediately lose stealth status again by being only behind a creature.
In a sense, this is balanced, since your rogue strikers could then just continue to camp behind your own fighters and shoot sneak attacks at enemies from just behind their buddies (since they don’t block for the player), which would make combat encounters quick enough, but also a bit boring.
Then again, as my player rogue pointed out, when there’s two huge dragonborn warriors pounding away at an enemy, how are they not supposed to be able to hide behind them? They aren’t 5′ wide, surely, but certainly bigger than a half-elf in every other dimension.
I just think that with a further update (yuck), we might be able to get a bit of clarification on the fact how allies grant cover, but cannot grant superior cover.