Here is another spell rules change - or at least rules clarification - for my Dungeon Fantasy / Dungeon Fantasy Roleplaying Game campaign, DF Felltower. For more, see my Revised GURPS Magic for DF page.
The various _____ Healing spells in GURPS have a simple but effective limitation: a -3 to cumulative castings.
It's come up in my games - is this successful castings, or just attempts?
For my DF campaign, I have ruled this is casting attempts. Succeed or fail, the very act of attempting Minor Healing, Major Healing, or Great Healing on a subject counts as a casting. If it fails, pay your 1 energy and take the -3 cumulative on the next casting. Succeed, and have the spell effect, pay the cost appropriate for that effect, and also have the -3 cumulative on the next casting.
This makes for a very effective limitation on total healing for the day, which in turn makes injury more of a resource drain than it can be in GURPS. It has played very effectively in our game, too, and made for interesting tactical choices about spell casting and strategic choices about healing spell planning. Pack your potions, the cleric can't just try and try again!