When attempting to run an import type migration using Insane Mode, either of the following messages are displayed:
The custom permission associated with the item could not be copied since it would exceed the list limit.
The maximum number of permission scope for this list has been reached.
This error is associated to Access Control List (ACL) limits. Insane Mode blocks the import in the case your migration exceeds this scope count. SharePoint has a limit of 50k permission scopes within a list. A scope is either an item, document or folder with broken inheritance. The ShareGate migration tool will display this error on the item, instead of migrating it with the wrong permissions.
Notes: Since the ShareGate migration tool runs the import asynchronously, doing a pre-check followed by a migration might not give the error on the same items.
The app can only identify this error if you run a migration with a 50k permission scopes. If you add 10k permission scopes in a list with a 45k permission scopes, the error won't be displayed.
There is no solution/workaround for this issue.