Personally, it hasn't happened to me, but it has with two fishing partners over the years. Such a shame, but in keeping with the regulations, one of the fish was released, unfortunately, still bleeding. The other certainly didn't seem like it would make it, but we made a small rock pool at the edge of the river and put the fish in it. Then informed ZEC where it was.
In Kamchatka in 1996 I tried to revive a beautiful 18 lb hen steelhead for a long, long time. Was unsuccessful. I was there as part of the Kamchatka Steelhead Project (KSP). Steelhead are in the Russian Redbook (endangered species) and are illegal to even fish for. The KSP, though, was a scientific research program where it was allowed. All our fish were to be released after measurements, scale/tissue samples/tagging, etc. were performed. The program allowed only 12 (I think) fish to actually be killed to allow collection of additional organ/bone samples. It was a real shame, a gorgeous fish, but she was delicious. The only steelhead I've ever killed.