I ran into a situation today where I had a beautiful plan, diagrams and naming convention all set to go, for a new 4 FARM SharePoint Implementation. I had planned on having two SQL Instances created, one called INTERNAL (Intranet, etc.) and one called EXTERNAL (Internet/Public, etc.).
The problem is, after creating the INTERNAL SQL Instance, I went to create EXTERNAL and discovered it was a reserved name/word in SQL. DRAT!
As such, I’ve since updated my diagrams and instances. But in the meantime, here is a reference of reserved words/names for SQL, so you (and I) can double check, before going down this path next time.
Reference: http://msdn.microsoft.com/en-us/library/ms189822.aspx