Another "fix" I made to GURPS Magic for my current DF game.
GURPS Magic takes two approaches to "reversible" spells.
Entombment - this spell costs 10 to cast, but also lets you cast the reverse of the spell for 6 points.
Flesh to Stone, Stone to Flesh - these are two separate spells. Stone to Flesh's only effect is to undo the effects of Flesh to Stone (or similar supernatural petrification).
I'm not a big fan of separate spells that essentially provide a spell and a way to reverse it. I don't mind if a given spell Summons X and another Dispels X, or one Destroys Y and another Creates Y. It's the spells that only counter another specific spell that I find to be a bit crufty. All Flesh to Stone does is reverse another spell; it's not separably useful. It can't animate a never-living statue. It can't turn a wall of stone into a wall of flesh to let you cut through it to escape a trap. It doesn't do anything except counter a specific other spell's effects.
So in my game, there is no Stone to Flesh spell. It's subsumed into Flesh to Stone. Cost to cast is the same, and you still need the same prereqs to cast it - so if you're lacking Magery 2, say, you can't reverse the spell. Essentially I got rid of one spell by taking Entombment as a guideline and putting them together.
What about clerics? The base DF cleric can only cast Stone to Flesh, not go around petrifying people. That's fine - they get a more limited version of the spell. Cost is unchanged, but then again, they don't need to provide any prereqs for it aside from the right level of Power Investiture, so it's fine. They get the spell but can't use it to petrify people, just fix petrification.
Why not Earth to Stone and Stone to Earth, too? Because neither is a pure counter to the other. The first will let you turn any old earth to stone, the other stone into earth. Neither is limited to only countering the other spell. The goal is to eliminate cruft, not merge all possible reversible effects.
Any other "this spell only reverses this other spell" pairs I'm missing? The is the only one that's come up in play, so far.