We were a mile and a half from home on a 30 mile ride heading up the last climb in our 30 - 23 gear when suddenly the resistance disappeared as our feet and pedals went down. I was trying to process what was happening and also get my foot unclipped in time to not fall down.
I thought at first that the freehub had broken but on a closer look I could see that just one cog had broken. We walked up the rest of the hill and then shifted the chain over to the working cogs and were able to ride home.
This was a 2nd generation 10 speed mega-range cassette from Santana 11-34 with the largest four cogs (the 23, 25, 28, and 34) mounted on their own spiders to save weight instead of each cog being a solid ring like the 1st generation 10 speed which came with our tandem in January of 2006. We put the new cog on about 10,000 miles ago.
Sometimes while standing to climb I have been known not to get a good shift and the chain will go back to the smaller cog with quite a jolt which happened a few days ago. I can't remember if that cog was involved but such an event puts a lot of sudden force. Anyway...ours broke.
Here's a couple of photos where you can see that it is obviously broken and there are some missing bits of metal.
I put the original cogset back on and we had a good ride today on the same course.