I learned that floors and walls are always magmasafe AFTER spending two in-game years constructing two large basins and a magmaeduct out of alunite blocks, almost going insane from the dwarven inability to construct walls ("creature occupying tile").
I don't see that as a major facepalm, unless you were low on magmasafe materials. Unless you meant cavern floors and cavern walls?
Constructed walls and floors are always magmasafe though, even constructed ICE walls.
Well, I somehow assumed the natural walls of my basins and magmaeduct would not be safe from magma, so I enlarged it and plastered everything with alunite floors and walls. Took me ages, and nothing useful got done because everyone was carrying stone blocks around and building walls/floors.
It also cost me two dwarfs because I finally found out the hard way when someone dropped the block of a deconstructed floor (above the reservoir, for magma workshops) into the basin. While dousing him and a nearby child in magma, the block also punched a hole into the floor and emptied the alunite basin into some leftover dugout below.
All the while I could have just used the dugout basin/magmaeduct as it stood and started everything two years earlier.
Also, there apparently is a way to make even guided minecart routes dangerous. If the dwarf guiding the cart is an off-duty soldier and gets activated while guiding it up some ramps, he will let go of the minecart - which will then immediately roll back into him. Though I'm not entirely sure if it was him guiding it or another squad mate a few z-levels up the route, a compound fracture in the hip and leg seems a bit harsh for a wooden minecart rolling down 1 z-level?