Nose hooking a carabiner is about what it sounds like: getting the “nose” of the carabiner stuck on a bolt hanger, stopper wire or webbing without the gate completely closing. Carabiners are extremely weak in this orientation and can break in a very modest fall or even bounce testing.

If you ever see a carabiner in this configuration, stop and fix it immediately.

Black Diamond did some testing on this, and carabiners failed as low as 2 kN, YIKES! (Side note, if you're a gear nerd and like to read about gear testing, the Black Diamond QC Lab Archives has some interesting reading.)

Here’s a link to those test results.

Nose hooked carabiner on a bolt hanger.

Nose hooked carabiner on a sling.

Fortunately, there’s a solution: use a keylock / notchless carabiner. The smooth, notchless nose makes it almost impossible to nose hook.

Just about every modern solid gate carabiner is made like this, but many wiregate carabiners are not. A few wiregate carabiners are notchless: the Camp Dyon, Wild Country Helium, DMM Chimera, and Petzl Ange (below) to name a few.

And, related to this, is the reason why the top carabiner in a quick draw must be allowed some rotation in the dogbone sling and not used with any sort of a rubber device like a Petzl String. The rubber device can prevent the top carabiner from rotating and result in nose hooking or (below), levering the carabiner at a bad angle .