A recent survey conducted by Harris Interactive reveals that roughly 1 in 10 IT professionals still has access to accounts from a previous employer. And, considering that this was a survey of IT people, it’s pretty likely that these accounts are privileged in some way.
Are your employment termination procedures up to date?
(The survey has some other interesting conclusions as well, though I would take them with a grain of salt. Most of them concern account and identity management, and the survey was sponsored by a software company that just happens to have products in that space.)