Error 'Mixed DML Operation' when you create or update records through Process Builder or validation rule

A Mixed DML Operation error occurs in Salesforce when a DML (Data Manipulation Language) operation on a setup object is mixed with a DML operation on a non-setup object in the same transaction. Setup objects are sObjects that control user access and permissions—such as User, UserRole, Group, and GroupMember. Non-setup objects include standard and custom records such as Account, Contact, Invoice, and custom objects.
Salesforce enforces this restriction to prevent data operations from executing with incorrect access-level permissions. For example, you cannot update an Account and a UserRole in a single transaction.
This error commonly appears in the following scenarios:

• A Process Builder or Flow that attempts to create or update a setup object and a non-setup object in the same transaction
• A Validation Rule that references both setup and non-setup objects through formula fields
• Custom Auth Provider implementations using a RegistrationHandler that updates the User object while the validation rule references a Contact field

Example 1 — Process Builder Mixed DML:
A Process on the User object is configured with an Immediate Action to create a record on the Account object. When a new User is created and activated, the following error appears:

Error: An error occurred at element myRule_1_A1 (FlowRecordCreate). INSERT --- INSERT FAILED --- ERRORS : (MIXED_DML_OPERATION) DML operation on setup object is not permitted after you have updated a non-setup object (or vice versa): Invoice_Statement__c, original object: User

Steps to reproduce:

1. Create a Process on the User Object
2. Set the criteria as Active equals True
3. Set the Immediate Action as Create Record on the Account object
4. Activate the Process
5. Create and Activate a New User

Example 2 — Validation Rule Mixed DML:
When a custom Auth Provider RegistrationHandler calls updateUser() and the User object has a Validation Rule that references a Contact formula field (such as Contact.AccountId), the following error appears at login:

"We can't log you in because of the following error. ERROR_CREATING_USER: Validation Formula 'UserProfileSwitchRule' Invalid (java.lang.IllegalStateException: Programmer Error: you cannot mix EntityObjects with different UddInfos within one transaction: id = 003**********"

Steps to reproduce:

1. Login to Community URL
2. Click Sign In
3. Login using credentials for an authorized community user
4. The error is generated