I failed to train my replacement
I was leaving a company which was cutting nearly half its staff as it was running out of money.
I showed the putative replacement the web control panel for the firewall whilst his boss observed. I made no attempt to explain in detail, just quickly shipped showed the key configurations like the vpn tunnels to explain which was which. There was a look of panic, and they asked whether all that was voodoo magic, but I said it was just everyday stuff, anyone should be able to do it.
They went off into a huddle. For the first time, the management realised my job wasn't trivial and maybe they couldn't find a cheap graduate to do it.