If you're migration a Dynamics CRM Organization from one on-premises instance to another via the SQL backup / restore method, as part of that process you're prompted to map users from the original organization into the new one.
Sometimes this is an opportunity to clean up old users who don't need to be mapped going forward. If you decide to do exclude users, or some can't be mapped, and those users happen to own Workflows or other critical components (think Business Rules), you must Reassign all the records that the user used to own. Ideally do this before you do the backup.
If you fail to do this when the workflows run you will likely encounter errors because the associated user doesn't have the required security roles. Or in the case of a Business Rule, you won't be able to import solutions that update that Business Rule and the import will fail.
So like anything, regression testing of functionality is important even with these 'holistic' migrations because it can reveal these kind of problems.