Any situation. Pick a weapon with or without a bipod, go prone on uneven terrain, and deploy.

Here's something else to think about. Shooting with a canted weapon should have a negative impact on the trajectory, ie in reference to the point of aim, the point of impact will shift low and to the side at which the weapon is canted. I did not observe this in the game however, which is a double-edged knife.

On one side, since a deployed weapon is canted on uneven terrain, well at least the player doesn't have to worry too much about it, since game's ballistic doesn't consider that factor. At least not from what I've observed.

On the other side, well, that's something that must be taken care of in real life. So I think it should be in a games that "features realistic ballistics". (See second item here: http://arma3.com/features/weapons)

It's not a show stopper, only a detail.

But at least if weapons would be level when deployed, then there would be no reason in the first place to think about a bullet's trajectory when the weapon is canted.