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.
Upgrade my PC. I put new parts in but I dont notice any real gains because my system was already running well.
My most costly mistake was probably installing gentoo on a Chromebook. That took so long and was both fun and extremely frustrating. It was working but now I’m getting these weird drive errors because the entire thing is loaded on an SD to avoid using the Chromebooks 15gb internal storage.