That's why they are called 'snap' rings.
Seriously, I've seen it happen, but I wouldn't call it that common.
Was it on your Compounder clutch assembly?
It used to be a fairly common issue on the early 41TE about 20-25 years ago. Some of the hardened snap rings are directional and must only be installed with a particular orientation (flat side down). A paint dab was used to identify thickness.
Was this under warranty I hope?
I don't think it's very common, certainly not to the extent that the 41TE experienced, but there have been a few failures here and there. It's basically a 41TE with an underdrive compounder attached, and that compounder seems to be what goes out the most frequently with early ones the most affected. The 4 speed part of it seems at least as reliable as the newer 41TE to me, and they were pretty decent transmissions after 2001 or so.
Dad's 2008 Caravan had a pretty catastrophic failure at 140k kms that I figure came down to either the compounder or differential, but once it got a replacement 2010 model transmission from a boneyard it was fine. That van has 410k kms now and is still going. I actually really like this transmission... wish my 2004 Pacifica had it.
A broken snap ring may not be the primary failure, but the result of another failure. A sudden unexpected mechanical force from something else failing nearby may exceed the strength limits of the snap ring and cause it to fracture.
Many times the snap ring over-stress is from the failure of an adjacent part.
I agree with imperial crown , I have seen these vehicles with many miles and no failures.they are mechanical things and can break.the last dealer I worked for was also a Honda dealership,they had transmissions that failed at 10 miles and problems just like any other car.there are millions of engines and trans built some of them will fail