I had a close call a few weeks ago when I accidentally clipped my lanyard to a NON-CLIMB RATED accessory belt loop. I have resolved to replace all such accessory loops with something else that is either climb-rated or so obviously different that no such confusion can happen again.
I was already putting some weight on it when I looked down and noticed that something wasn't right. It is just stupid -- there is no good reason to have any weak belt loops. Tubular webbing is cheap and strong.
Glad you're still around to tell the tale. WhenvI was first learning to climb, my foreman told me to NEVER trust what you hear when clipping in. ALWAYS look and be sure of what you're clipping in to. I was doing line clearance at the time, and our saddles had steel side D's. So it was possible to hear what sounded like a good connection. But then you look at it and see your biner is not shut. Dicey stuff indeed. When I rec climb I use a Tengu. It has soft side D's and you can't hear anything when clipping in. So you have to look.
Yes, I had grabbed the type of biner that I wanted and clipped it onto the lanyard without noticing that it wasn't really the intended biner. It was a spare. Also as you say my D's are metal and the accessory loop was metal so it "sounded right."