I recently had to debug some code that threw the 'This entity is already locked' error message (code 80043B1D) when updating an order product (salesorderdetail) entity.
This error can occur even if the state of both the order and order product allowed editing. The error occurs if the productid property is set on the BusinessEntity that is passed to the Update method. A general principle when coding with the CRM platform is that, if a property value is set on an entity passed to the Update method, then the CRM platform will assume that the value has been changed.
In this case, if the productid is passed, CRM assumes that you are trying to change the product, which is not permitted if the ispricelocked attribute is set to true on the order. So, the solution is to always ensure that you know which attributes are actually changing, and ensure that only these properties and the primary key value are passed to the update method.