In this photo the carabiner is radically tri-loaded.
An impact on such a carabiner could cause failure.
One could tie the sling off with a pre-equalized knot, but this isn't required. The following photo shows one quick example of a tie-off that eliminates the possibility of triaxal loading.
Triaxal loading is a detail that a lot of climbers don't think about. But it is just these kinds of minor details that can get you in the end. The phrase, "the Devil's in the details," didn't come from nowhere.
--Jason D. Martin