In order by how quickly they appear:

Users can’t log in – happens immediately. Most visible failure mode
Permissions silently disappear — users can log in to SharePoint, but they can’t access the files, applications, or SharePoint sites they need
Service accounts stop working. Automated processes and integrations break down quietly until someone notices output that doesn’t arrive
Group memberships are lost: email distribution lists (security groups), Teams membership and Teams membership are all affected.
The conditional policy is not being applied correctly. Users are either denied access to resources which they should have access to, or they gain access resources that they shouldn’t.

One of these is an incident. Two or more of these happening at the same time is an incident.

Identity migration needs to be carefully planned and validated, not just run in the background as a “background task” while the “real work” of migration is being done.

#Migration #Identity #AzureAD