Contacting Ex-Employees
The question comes after I read an article from Ask a Manager about an employee who was being harassed by their old boss 16 months after leaving their old job. This is an extreme circumstance (it seems like the boss is an extra vindictive person) but it should prove a point that while it's okay to contact an old employee a few times, it should be done sparingly and it should be done with a great deal of gratitude.
In one of my old jobs, my predecessor left for another job within the company (at a new location). He was a really nice guy and had a hard time saying no to anyone so he let everyone know that he was available to help when he left for his new job. Well the problem was that everyone wanted to contact him and they wanted to contact him all the time. It not only made him feel that he really had never left his old job, but it also was really frustrating for me because--correct or not--it always made me feel that no one had trust in me to make the right decisions in these situations.
It's always nice to have old employees who are readily available to help, but it's always worth understanding that the person who left is under no obligation to actually help you--and it could be doing a lot of damage to the confidence of the people left behind. It's okay to contact old employees but, as they say in alcohol commercials, if you choose to do so, do so responsibly.