At project start we have looked for a specialist in a particular IT area. The project owner found one (using his managerial channels) – the specialist was persuaded to work at the project for half an year ( as “temporary solution” for starting the project). We were not informed about his interim acting, he was not direct member of project team. After one year he stand up and announced to stop.
We had to slow down the rollout and in a short time to train two another engineers in this particular system.
1. Do not fall back that you have all project role covered now, check their availability for future, their “buy in” in the project and manage it. 2. Verify the information you got, even in case of the project owner (the project was very complex , project owner was deeply involved in it and forgot about the limited support of mentioned specialist).