In my opinion, the IS_MEMBER() function is not properly documented.
The issue is: If you are a sysadmin, then IS_MEMBER('custom_role') will ALWAYS return 0, regardless if you are member of 'custom_role' or not.
This is not clearly documented. We relied on the documentation, and now we need to rewrite stored procedures to handle the special case when someone is sysadmin.
https://msdn.microsoft.com/en-us/library/ms186271.aspx
- Edited by Georg Scholz 18 hours 8 minutes ago