In our line of work as professional dog walkers we deal with a lot of leashes. (Over 200 per day to be precise.) Leashes need to be replaced from time to time due to wear and tear. Specifically, the leash clasps will eventually become worn and misaligned with repeated use.
Last week Minder Shannon had TWO of her client's leashes malfunction. Shannon was able to quickly grab both dogs (two in one week!), but it could be tragic. Upon inspection both leashes exhibited the same problem. The "bar" was no longer aligned with the loop.
This is one of the more common leash failure issues and it's difficult to explain without seeing it actually happen. The leash clasp will still "work," but eventually a dog will move in just the right way and the leash will pop off. How? When the bar is off center, it allows force from the collar ring to push down on the bar, releasing it. (Normally you'd have to use your thumb to press down on the release knob.) In a non-compromised leash that bar would be aligned with the loop, and only a finger depression on the release mechanism would on the clasp.
Faulty leash clasps will usually will pop off when a dog is excited / bouncing around or shakes. If you have any leashes that look like this, just purchase a new one. Don't wait for it to happen!