For me it is not recording credentials with the assumption I would simply remember them later, while having every opportunity to archive them before eventually forgetting. Also, not keeping detailed enough notes & photos of exactly how my hardware is attached.
I installed Ubuntu back when that was popular, and insisted on having all the graphical bling, like 3d cube that would spin to change desktops. And windows that shook like jello when you moved them.
Of course all this messing around by an amateur did nothing for stability and after 3 or 4 frustrating issues I went back to Windows.
I can’t even function without the Compiz 3D cube anymore, it makes it super easy and visually intuitive to switch desktops. Very handy for someone running 4 virtual machines simultaneously…