Last week, I deployed one of the Web API website that uses Entity Framework to production server. In the publish wizard, I generated database connection string for production so that it'll connect to production database when deployed. To my surprise, few API calls resulted in an error :
System.InvalidOperationException: There is already an open DataReader associated with this Command which must be closed first.
Yup, the solution was to include MultipleActiveResultSets=True; in connection string. In fact, this was there in the connection string of my development database but show how not included while generating connection string of production database.
Hope this helps.