The picture tells the story. Early 80s Schwinn lug frame. I went shopping last weekend, on the way home, heard a "clunk" as I was getting on the bike. No need to explain how I figured out that the fork end had broken clean off. Other side was still together, but cracked in the same way.

The only "analysis" was that it looked like the cracks went right through the corners of the rectangular holes in the fork ends. Those holes were for a secondary retention scheme that Schwinn used on some bikes. I prefer the Brilando clips, but whatever. I'm not an engineer, but I have a hunch that the corners of the holes could be stress risers. Two other old schwinns in the family fleet have those holes, another does not. I've inspected the forks closely for any signs of cracking.

I suspect the crack developed slowly, and I might have noticed it, had I done a more frequent inspection. This bike has been ridden hard for about 20 years, and I don't know how it was used before that. The fork was always a bit crooked. Was it crashed in its former life?

PSA: I think it's worth inspecting old frames for mechanical integrity on a periodic basis. I don't know how often is necessary, maybe some of the experts could advise.

Good news, I had another Schwinn frame hanging in the garage, took about an hour to transfer the parts, new bike day!