Despite the fact that I have worked on a lot of Dynamics CRM implementations over the years, even occasionally I will make a “Noob” mistake. Sometimes even twice.
A few months ago one of my clients wanted to re-assign many of their accounts to different sales people based on a new territory setup. This process is easily accomplished in Dynamics 365 a number of ways.
My client also has a separate team of technical support folks that will own support cases in CRM as well as operations people who will own orders and invoices.
When it came time to re-assign the Accounts, I used advanced find based on the new territory assignments, selected the Accounts and hit the “Assign” button.

Now if I were a new to Dynamics 365 consultant, fresh out of school and ready to take the world by storm and made this rookie mistake, then maybe it could be forgiven (like when I advised this for a CRM 3.0 customer way back when…)
However, I am now older and supposedly wiser.
For those of you don’t see the danger, what happens “out of the box” is that when you assign an Account to a different user, the related entities that are linked to these records as child in a “Parental” relationship also will get re-assigned to that new user.
Things like activities, cases, quotes, orders, invoices.
All of a sudden, the technical support folks no longer saw any cases in their views. The operations folks had no orders or invoices.
Worst of all? Sales people started seeing support cases assigned to them! (Sales people don’t want ANYTHING assigned to them except big fat commission checks.)
There is no warning, no danger signs and no easy “undo” button.
The actual fix going forward is quite easy and ideally should be incorporated in your initial configuration of Dynamics 365.
Go to Settings -> Customization and ideally create a solution and add the Account to it or whatever method you are using to manage configurations withing Dynamics 365.
Expand the Account, locate the 1:N Relationships and group anything that has a “Parental” behavior type. Open each relationship where you would rather NOT the child record reassigned to the owner of the parent record. Note that for some relationships you might prefer this reassignment to occur.

Next, change the relationship type from “Parental” to “Configurable Cascading” and modify the “Assign” feature to “Cascade None”.

Repeat the process for each relationship you want to change. Repeat the entire process for the “Contact” entity as well.
Going forward, when an Account is re-assigned, the related child records will remain under the current ownership.
This is mentioned in the Dynamics CRM Customization and Configuration exam, but sometimes you need to touch the stove to realize its hot.
I hope this nugget of wisdom helps out someone down the road, or serves as a reminder to me.
Merry Christmas!
Nick