When removing a user from the frontend, while that user is owner of entities such as a screen, map or dashboard, Zabbix is throwing the error:
"User "USERNAME" is <entity> "<entity name>" owner. Cannot delete user.

It would make sense not throwing that error, but instead migrate the ownership to the logged in user or any other user of choice. With just 1 entity owned by the to be removed user, it's no problem to manually migrate the ownership, but with 50+ entities.