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’ve been running Linux since 2011, starting with our data recovery and antivirus scanning system at the computer repair shop I was working for at the time.
Even my boss didn’t understand why I wanted to install Linux. Keep in mind, this is back when the TDSS/Alureon rootkit was going around on Windows systems.
I explained it like it was, that if our main backup/antivirus system was running the same OS as the infected computers coming in, then it was only a matter of time before our main system got infected.
So, he accepted my advice and let me set everything up. More or less just the bare basics really, smartmontools, gparted, firefox, google earth (just because), and a few other relevant programs to help with our daily tasks.
Then, one day when I was off work, a new employee decided to install some plugin into Firefox to share bookmarks and stuff across different devices…
Somehow, he borked the main tech user account, it wouldn’t even login to the user interface anymore :(
I had to spend a few hours, with the skeptical boss over my shoulder, waiting to see if I could get the system back running right again.
And so I did, while learning lots of new things at the same time. When I learned the hotkeys to switch to other terminal sessions, then I figured out how to create a new account, erase the old account, and get logged back in and running.
The customer data backup drive was separate and detached through all that, so customer data was safe the whole time.
The boss almost said fuckit, reinstall Windows, but I was persistent. And that system helped salvage over 200 systems with the TDSS rootkit, which would have almost certainly doomed our backup system if it was running Windows.
I told that new guy to never fuck with my operating system setup or configuration again, at least not before consulting me and getting approval or even assistance first.
When you got a bare minimum of the past 100 customers’ data backed up and virus checked, you don’t dick around with the main backup system.
So, honestly, I can’t think of a single truly costly mistake that Linux has cost me…
As far as that other employee that messed it up for a bit, well I dunno, it wasn’t too long after that the boss fired him…