It is not always disgruntled staff that cause a company problems but companies should always be on their guard from them.
A recent survey suggests that 88% of computer administration staff would steal data if they were fired. That is a startlingly high proportion, and a problem that can be easily overcome by employing a suitable policy of changing passwords when staff leave, whatever the circumstances.
The fact that computer administrators are much more aware of the networked environment and often have remote access privileges, wheter authorised or unauthorised makes them the highest risk of all.
Any company contemplating firing a member of staff that has elevated privileges would be well advised to seek help from a security professional to ensure data leakage is minimised prior to any administrative action being taken.
Source: http://www.theregister.co.uk/2008/09/05/88percent_it_admins_would_steal_sensitive_data/