I was recently advising a transition project where a customer was switching support and maintenance supplier. This means that one organization must take over a system that has been maintained by another organization for a number of years.
A lot of information is lost in these transitions because knowledge of the problem domain has been accumulated in the heads of developers over many years. This loss cannot realistically be mitigated.
But sometimes, specific information about the technical implementation is also lost, and that should not be allowed to happen. If you are receiving code to maintain, you need to make sure that you can check the source code out of version control, make a cosmetic change and follow the build and deployment instructions to deploy a new version all the way to a pre-production environment.
I’ve never seen a transition where this was possible right away. Trust, but verify.