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Apex Test Failure differences between Deployments and UI due to 'MIXED_DML_OPERATION' errors

Knowledge Article Number 000205689
Description Customers can sometimes see a difference between the number of Apex Test failures when running all tests via the UI compared to the failure count observed when performing a deployment. A comparison between the differences leads to the conclusion that the differences can be accounted by observing 'MIXED_DML_OPERATION' errors in the UI but not in the deployment, why is this?
Resolution During deployments we do not run mixed DML operation checks, whereas via the UI we do (either 'Run All Tests' or running a test for an individual class). If the differences between the test failures seen in the UI compared to the deployment is solely down to 'MIXED_DML_OPERATION' errors, then this can be deemed expected behavior as we intentionally do not check these in a deployment.

***NOTE*** Same behaviour can
also be observed when simply running tests outside of a deployment/validation scenario via the Metadata API, again this is expected behaviour


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