I went to put in a Moen faucet that had the supply lines already attached: 3/8" compression connection at the end of them which is very typical application. It turns out the shutoff valves I had in there were 1/2"compression on the end. Darn! Went to get some adapters to go from 1/2" compression to 3/8" compression and one of them dripped so I changed out the entire shutoff valve, and was on my way.

You see a lot of shutoff valves in kitchen or baths that are 1/2"? Seems like most are 3/8". I think the reason the adapter leaked on one of them is because I was in a hurry and messed up the rubber o-ring that came with the adapter. Is that what makes the actual tight seal connection?

probably the rubber ring was the seal.
always see compression made with brass ferrules.
in all my career i have never installed a stop with 1/2 inch OD outlet.
pretty rare fitting.

That surprises me, packy, that you have not come across 1/2 in compression outlet supply valves. I come across them in Toronto sometimes and it is always feeding a LoBoy toilet using a 1B1X fill valve. I think Kohler, Eljer etc recommended 1/2 in compression supply be used with the LoBoy toilets because they wanted the high flow for effective bowl wash.

i've come across a few in that situation but you could count on one hand the number i have seen.