Remember to disable/delete the account for users and admins who are leaving.

Happened at a company I used to work at, I was in the office while having a car service just after they had sacked one of the main Unix system developers that morning, all of a sudden applications and services started dropping, people getting logged off and unable to log back in, processes were disappearing before your eyes. My supervisor was there with me and he twigged what was going on, ran to the comms area and ripped the modem cable out of the wall - fortunately they'd learned from issues a year or so before and had a 2nd system acting as a transaction mirror, recording all the database changes - they were off line for about 4 hours until the main data was roughly replicated back but there were still some users not able to log in a few days later. Part of my job a few years later was often to find the sysadmin root password and any other alt accounts they might have created for themselves, then at a designated moment to change these passwords (often before disciplinary action), it was cloak and dagger stuff and often involved some distraction techniques from the client - we suspected one had an alternative account he made for himself but we found his root password easily, changed it and made an excuse that a manufacturer update had overwritten the password and it would be a day or two until we can reset it, then we found his alt account when he logged into it, got him away from the terminals, changed that and then they dismissed him and escorted him out the building. Fortunately every time we had this scenario they were dumb enough to log into any alt accounts they had, one chap had 3 but most only had 1, even better that they never suspected me or my colleague were tapped into the connection and logging all their keystrokes - I was talking to one admin while we were drinking tea, my workmate was across the corridor giving HR the thumbs up that we'd got all his accounts and passwords, he was actually boasting how he'd made himself a "secret" user account in case he had any disputes and he would be able to dial in during the night and wreak havoc, nope, didn't happen, we found the account very easily and messed his plans up.