If you use the tSQLt unit testing framework for long enough, you will eventually run into the “An INSERT EXEC statement cannot be nested” problem. This problem happens when you try to insert the output of a stored procedure into a temporary table for validation, but that stored procedure internally already does the same thing, inserting the output of another stored procedure into a temporary table. SQL Server does not allow this.
Researching the problem will lead you to a lot of articles and posts in which people say that you must change your code, and for example use: