The scary wake-up call for me when I heard the story of someone who informed their manager of their 2-week notice. And instead of having 2 weeks to sort everything out, they got locked out immediately.

Our business has to do this for security reasons upon learning an employee is no longer going to have an ongoing relationship. However, we explain this to the employee ahead of time, and the two weeks are still paid - the employee just doesn't get to touch sensitive information anymore.

I began disentangling things from my previous employer about a year before I actually left. 2 weeks notice would basically be a formality, I'd have effectively quit months before anyway.