I'd recently finished my magma-based automatic waste disposal unit. Three minecarts working side by side to bring refuse and unneeded stone up over a magma pipe and dump it in. The 3x1 dumping hole is covered by three 1x1 stone bridges connected to pressure plates to ensure that the chute is closed off when not in use, so as to prevent unwelcome intrusions through the waste chute by surprise visitors.
After a while I notice that the middle bridge is somehow stuck in the open position. This calls for immediate correction, so I order the bridge dismantled and rebuilt. The only dwarf in my fortress who can do this is my legendary mason, being also the only architect. After the final order is given, I focus my attention on a different matter. (For some reason the minecart bringing down iron ore to the magma furnaces keeps dumping it on the final ramp instead of at its destination. Why is that?)
While occupied with this, a report comes in: the magma sea has been discovered! Strange, I don't have any miners working that deep at present... A few ticks later, a new report: "Raw adamantine has beeen discovered! Praise the miners!" I zoom in to find that an adamantine pylon has indeed been discovered in the magma sea.
The facepalm moment occurs a week later when my legendary mason is reported missing and I go to the waste disposal unit to find the middle bridge has only been halfway completed. Two lessons to learn from this:
1: Make sure to shut down the magma-based waste disposal unit before sending in vital personnel to perform maintenance.
2: Magmanauts can be used to discover the location of adamantine pylons.