When one of my players was making his latest mage (Dryst, who replaced Nakar the Unseen, who replaced Volos the Too Close To The Minotaur), he asked about the Concussion spell.
It's always a bad sign when my players start a sentence with something like, "Oh, yeah, Concussion. Are you really allowing that spell in the game?"
I said, "Yes, I was, why?"
Long story short, we ended up changing the spell.
At first blush, the spell appears to be a combination of an explosive missile spell with Thunderclap, which can potentially stun or deafen targets.
At a closer look, it's significantly more powerful than either Thunderclap or most other explosive missile spells, but also has some weird non-effects.
It's a standard missile spell, in that it does 1d crushing/2 points of energy. Plus it's a HT-3 roll within 10 yards or be stunned (roll HT-3 per second to recover). Yet oddly, no deafening.
Combine that with tunnels, where sound carries extremely well (see GURPS Underground Adventures, p. 10), and it becomes a hot mess of calculations of blast area and tons of nearly-automatically stunned opponents.
We decided it was a little too much oomph - that stunning effect was much more than Thunderclap, yet the sound and pressure couldn't deafen you. And Thunderclap can deafen but not stun, which we thought was weird. Especially since the prereqs are Shape Air and Thunderclap - armed with those two spells you can learn Concussion and create a greater yet lesser sound effect than Thunderclap could on its own.
I really hate that kind of thing - you can see it in my Enslave ruling where I ruled that you can't see through the target's eyes and ears unless you've got the appropriate Rider Within-type spells, since none of the prereqs provide that ability. You've got to have a solid underlying way of creating the effect (or a lesser version of it) to make prerequisites make sense. And you've got to have a solid way of making separate effects happen before you can combine them into a spell.
So here is our revised effects - yes, nerfed effects, if you prefer:
Thunderclap: Effects as listed on GURPS Magic (p. 171), but if the HT roll to avoid deafness is failed by 5 or more, the subject is stunned (roll HT each turn to recover). Protected Hearing gives a +5 to this roll.
Concussion: Acts as a Thunderclap spell out to 3 yards or the maximum area of the explosion, whichever is greater; area is doubled indoors to 6 yards or twice the maximum area of the explosion. The HT roll to avoid deafness (and possible stunning) is at -1 for ever 2 HP of injury suffered.
So Thunderclap is upgunned a little to more than a simple "deafen with no other effects" spell. Concussion is downgraded from a somehow non-deafening but nearly automatic stunning effect in a 10-yard radius (or bigger, if you read it to mean within 10 yards of the damaging area, which makes sense if a bigger blast means a bigger blast area). Instead it's a damaging area spell with a linked Thunderclap, and your chance of stunning is linked to damage just like with the Lightning spell's surge so we don't need a separate mechanic to do the same job.
I've learned to always respect the warnings of my players when they find something they feel is abusive. This is one of them I heeded before it did annoying things in our current game.