Was building a pump stack for a magma cistern. I remembered to channel under the impassable tiles so there was proper power transfer, etc, but for the pump output tile, rather than channelling it out from above to allow the above pumps to take it, I ramped the hole out from under - nothing special, it's always been my standard designating method for stacks. However, this time, I made three mistakes. The first was to ignore my usual OCD tendancy to remove all loose stone immediately to try and get myself to loosen up a little. I figured the magma would eventually melt all that stone away anyhow. The second was to not remove the ramps, which I didn't for the same reason of actively trying to rebel against my obsessive habits. The third was not walling off the space above the output tile of each pump layer. Why bother? It's not like any dwarf was going to jump in or anything.
Cue an hour later, real-time. I'm suddenly spammed with dangerous terrain cancellations. Thinking my power plant had sprung a leak, I zoom to the last dwarf to cancel and found them at the access tower for the pump stack. Why? Damned if I knew, but they were on fire, and died shortly after. I doublechecked the job cancellations to try and solve this mystery, and finally found it.
The dwarves were trying to clean the molten rock from the magma-filled pump stack.
The final destination of the magma hadn't been filled yet, so the pumps were moving fast enough that there was never actively a 7/7 block of magma in any of the tiles. However, the rocks got hot enough to switch into the 'molten' phase, and since I'd left the output space accessible from above, the dwarves were trying to path into the perfectly safe area to take the non-forbidden molten stone and clean it up. I immediately whipped out a d-b-f, but it was too late for the first few victims. They died in the heat while their comrades were carrying them to safety.
tl;dr I love Dwarf Fortress.