First time we've done that. We were launching from a stop sign, seated, but I was maybe pushing a bit hard, and the chain just snapped. It's a Nashbar chain made for long-chain uses such as timing chains and recumbents. I think maybe I was trying to get too much use out of the chain, assuming that a timing chain would last over three drive chains. I checked my records to see when it was last replaced, and I was quite surprised to see that I had over 9,000 km on it (I usually replace the drive chain at between 3- and 4,000 km).
I would think that a timing chain would not get a great deal of stress due to its simple, direct run. But thinking about it, perhaps the strength differential between my stoker (female casual rider age 69) and me (male ex-racer age 59) may have had some effect. That, plus some chainline misalignment - the front timing ring (Shimano) sticks out a bit further than the rear (Sugino), but not noticeably. Still, there would be some bend to the timing chain.
I will have to rethink timing chain replacement, start getting serious and checking it with the measuring tool instead of waiting every n drive chains to replace.