Shooting at a unit behind a unit that has gone to ground.
Let us say you have four units, two units which belong to play A, Units 1 and 2, and two which belong to player B, units 9 and 0.
It is Player A's turn, and it is his shooting phase.
His two units are side by side, whereas Player B's units are one behind the other (for our example here, assume Unit 0 is behind Unit 9)
Player A's two units both have line of sight to Unit 9, and they can still see Unit 0, but Unit 0 would be granted a cover save because Unit 9 obscures them.
However, Unit 1 is a unit with some kind of pinning weapon, be it a barrage weapon or a sniper or what have you. Whatever it is, it causes a pinning test. It fires first on Unit 9.
Unit 9 is forced to take a pinning test. It fails. They go to ground.
Is Unit 0 still obscured?