df says your disk is 80% full, but du says you’re barely using half, so one of them is lying, and it’s probably not the one you think.
You’re debugging a full disk alert at 2 am, and the df command is showing red while du / comes back looking totally fine. You run both commands 3 times, thinking you misread something. The numbers don’t match, and now you’re doubting your own terminal.
This happens on production Linux systems more often than you’d expect, and the fix is usually just 2 commands away once you understand what’s actually happening.