2 columns in ACT_HI_DETAIL_ : PROC_INST_ID_ and EXECUTION_ID_ are currently defined as NOT NULL.
When creating a task through the API, outside the scope of a process instance and with history=full you will get an exception as the system attempts to insert null in the above 2 columns.
FYI - I have dropped and re-created the table allowing the columns to be null in our environments for a week with no adverse affects. I will state an obvious disclaimer that there will be functionality that we are not using that may require these columns to be populated. I haven't checked the use of the columns in the queries.