Hi

A similar thing can happen when using a daisy chain. With the 'biner in the end of the chain it is perfectly safe but when the daisy chain has been shortened by clipping the 'biner at the end back though one of the loops of the chain, it becomes unsafe as the only thing holding you now is the tacks between the loops and not the sling itself. This is still safe under a light static load but under a dynamic load such as holding a fall it can easily break.

There is a video of this here

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Happy climbing
Nic

I've discovered (almost the hard way) that it's actually really easy for your draw to end up attached to just the retainer. That happened to one of my extended draws while in my rucksack. Fortunately my belayer / seconder was awake (I obviously wasn't ; and although it was unlikely to take a dynamic loading (it was being used on a non-hanging belay, with the other sling being an extended sling with no retainer and two locking biners. Not an ideal set-up overall, admittedly, but I wasn't in possession of enough locking carabiners to do the job).

Moral of the story - either don't add retainers, or check them properly every time you intend using them. I know I'll be doing that from now on; and if I don't, my climbing partner sure will!