For me, it was getting a handle on rsync for a better method of updating backup drives. I was tired of pushing incremental changes manually, but I decided to do a bit of extra reading before making the leap. Learning about the -n option for testing prior to a sync has saved me more headaches than I’d care to enumerate. There’s a big difference between changing a handful of files and copying several TB of files into the wrong subfolder!
I’m lucky I manually ran a few jobs before I started using rsync in scripts. When I didn’t think things through, I saw the output in real-time. After that, I got very careful about testing any scripts and accounting for minor changes in setup.