You’re modifying setup and non-setup objects in the same transaction.

Problem Explanation

Salesforce separates setup objects (User, Profile) from standard objects to maintain system integrity.

Root Cause(s)

1. Creating Users and Accounts together

2. Updating Permission Sets alongside data records

3. Test setup not isolated

Step-by-Step Solution

1. Move setup object DML to System.runAs()

2. Separate transactions using @testSetup

3. Use async Apex for one side if required

Edge Cases & Variations

1. Permission Set Assignments count as setup DML

2. Community Users increase complexity

Common Mistakes to Avoid

1. Creating users inside main test method

2. Ignoring setup vs non-setup distinction