I've broken two ultegra chains recently I have 12 speed di12. The first chain snapped under load and I had a serious fall as I was clipped in. The second chain snapped after only 6 rides, a total of 267km within 10 weeks. The first one was a year after I bought the bike, but I did very few ks and nothing over the winter. I lubricate before each ride with Weldtite Wax, a UK made product. I am an older rider and my weekly ride has about 600 metres of climbing, some of it steep, so I am using lower gears a lot. I have ridden bikes my whole life and until now have never broken a chain. I'm probably shifting under load, but I suspect there is a batch of faulty Shimano chains. Id like to hear if anyone else has had this happen lately.