When people leave, it is always painful but there is also a good side to it.
Not only do you lose a lot of knowledge, you also have a disruption to your services and you need to spend time on recruitment for a replacement. The good side is that it brings many practical problems to the surface that this person has been solving without the rest of the team and the manager being too aware of. Though I pride myself that all my systems normally run without interruption, reality is that there are always little glitches that are effectively dealt with by my team. 'Effectively' in the sense that the rest of the team is not always aware of it and that the respective person resolves the issue when it occurs. These type of issues are interruptions and should be resolved with a permanent solution instead of the repeated workaround that the team member or the team is actually hiding for the outside world.
When staff leave these issues become very visible and can become very painful if no handover took place or the person 'forgot' to pass the relevant information on.
It is not always that they leave the problem because they are lazy or incompetent. Sometimes the effort for the permanent solution does not weigh up against the acumulated effort for the repeated workaround.
Though I think it is good that these issues surface and do get resolved. The ongoing interruption remains an interruption no matter how small it is. Systems must be robust and should not rely on a knowledgeable person to keep them running. I have heard of cases where these ongoing manual fixes were maintained but became disastrous when the respective staff left.
When somebody is about to leave the team or already has left, it is a good moment for a stock take of outstanding work and of imperfect solutions to fix it all up.
Regardless of the above described situation, I find it always interesting to see how much people do that you just take for granted. When you sit down with a staff member and make a detailed list of what they do, you realize how much you depend on the unique knowledge and skills of the person.