Discussion on:

Delays in terminating network access to departing employees common

We had similar problems of forgetting to disable an ex employees logins. Luckily we've not had any problems because of this.
But because I'm paranoid about the security of my (yes MY..it's work but it's mine) network we now have a form for someone who's leaving to complete and we have an IT checklist of everything that needs to be disabled or changed right down to notebook boot passwords.