Nobody's Fuel
After a long hiatus, I came back to a fresh install of KSP and decided to start off a sandbox mode session with some fresh designs rather than resurrect my old saves.
Surprisingly, my first attempt actually made it into orbit. Inspired, I began my usual process of sending up fuel tankers and linking them together with docking hubs to create basic space stations.
After creating fully-fueled cruciform stations at 500km, 5Mm and 25Mm orbits above Kerbin, I realized/remembered the nuclear engines I was using for all exoatmospheric flight don't use oxidizer. And here are all these space stations loaded with more oxidizer than liquid fuel.
Kaboom! All stations deleted. Goodbye everything, hello next round of designs omitting oxidizer and featuring a LOT more exoatmospheric delta-v.
10/10, would screw up again.