Contact.LastModifiedById.getDescribe().isCreateable() returns true

Question

How is it possible that the following code can return true

Contact.LastModifiedById.getDescribe().isCreateable()

It was my impression that the LastModifiedBy field was never createable

Answer 1

In my dev org this apex:

Schema.SObjectType.Contact.fields.LastModifiedById.isCreateable()

Evaluates to true. There is a black tab option that salesforce support can enable that does allow audit fields like LastModifiedById to be creatable (but not editable). I strongly suspect this was enabled in your org when you checked this.