I did that once! I got the filler to flow, too! (tho not well.) I was just making a thingum to hold a braze-on, one of those "maybe I'll use this again someday but likely single-use" ad-hoc tools. I hadn't brazed in a while so I forgot which flux was which. I picked up a yogurt tub, that flux had been decanted into because the original jar it came in cracked. The flux looked gray so I though it was for brass, turned out to be just dirty silver flux.

I figured out early on that it was the wrong flux, but sort of brute-forced it. The joint is a bit ugly but plenty strong enough for the application.