I create an anti-goblin device involving a massive magma cistern over my entrance. The path goes over Iron grates surrounded by Coke roads and drains into the pit that an underground river drains off into. I see a trade caravan being followed by goblins, so I get ready to test it...and set the damned thing just a bit off too early.
Results:
-Caravan go boom! (was full of booze I ordered)
-Goblins and caravan are burned away, so leaving only valuable meltable items behind. (This was before I learned about mass-melt/mass-dump)
-I learned that constructions made of coal can be made to ignite, creating awesome smoking, burning constructions.
-I also learned that magma does not travel horizontally very quickly, and my rather long drainage tunnel quickly backs up, flooding the entrance to my fortress with magma.
Results 1 and 4 were the facepalms.
Result 2 was the nifty result I was going for.
Results 1 and 3 where awesome. Result 3 was so awesome that I have created several fortresses in an effort to recreate that result, and I'm still trying to figure it out. I know it can be done, but not how. My current hypothesis is that while constructions are temperature proof, they are not fireproof. Therefore, while you can use coal/charcoal/coke to store magma, if you use magma to set a flammable object (shirt, boot, elf) on fire while it's on a coal floor tile, the fire will spread.