My broken frames are all clustered around 2001-2003, when superlight aluminum alloy frames were becoming popular.
Rocky Mountain made a road frame back then, the Turbo. The regular one used Easton 7000 series aluminum, but they also did an Easton Scandium version circa 2000-2001. This frame was super light, had a nice ride quality, but unfortunately Rocky did not have much luck with them. They welded a chainstay bridge, and I had 2 frames crack at the drive side of this bridge, both at about 3500 km. There was clearly too much heat on the relatively small chainstay leading to brittleness.
The only other frame I broke was a team issue frame from a big company, but it wasn’t built by the big company, but instead by a local framebuilder Dion Cycles (long gone, but had some fame in the triathlon world briefly). This was 2003 and I was riding on an elite amateur team. The frames were Easton 7000, but they had an Easton carbon rear seatstay and used Easton dropouts. Problem was Easton made the dropouts way too light and used countersunk bolt holes for the replaceable rear derailleur hanger that were too big for the amount of material there. The frame would start to crack at the holes, then propagate forward and eventually crack circumferentially where the chainstay was welded to the dropout. Mine let go in a race, every time I would freewheel I would get a loud clank that I thought was a freehub issue. Got a wheel change, still made the same noise, then had a couple of instances where I blew the chain off the big ring for no reason so I stopped and then noticed the rear chainstay was completely detached and only chain tension was keeping it together. The banging I was hearing when coasting was literally the stay smashing into the dropout as it flexed. I think of 6 of the 8 frames failed in the same fashion.
My only other frame “failure” was a Colnago C60 rim brake frame from 2014 that developed a small crack at the exit of the rear brake cable in the top tube by the seat cluster. It was mostly cosmetic, and I had it repaired and then sold it. I honestly disliked that bike on every level and it was the last carbon bike I have ever purchased.
FWIW I have never had a steel or titanium frame failure in 30+ years of riding and owning many of both.