Connecting to a file geodatabase in Catalog with a UNC path and with a mapped drive can result in locking issues.
This violates the single instance model by opening the same database though two different paths. So the second one fails in lock validation.
Allowing both to be open breaks editing, as edits on one are not be seen on the other. Attempts to write on both could lead to failures, or to writes from one being overwritten by the other.