A "Can't see the tree for the forest" problem.
During a break in a C# class I was delivering last week I typed up a little demo for accessing SQL Server, and got an error. A quick read of the error said that it couldn't find the server, and it hinted at a protocol error.
Additional information: A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: Named Pipes Provider, error: 40 - Could not open a connection to SQL Server)
Seeing the big "Named Pipes" tree standing there… I Binged and Googled and found all kinds of stuff… that did not help.
While the answer to the problem was clearly stated in the message above, "Verify the instance name is correct", I didn't see it as I was looking at all of the other "trees" in that little code forest. The "tree" that I needed to deal with in this case was a C# beginner error of putting a backslash in a string. (I copy and pasted it without looking at it!) The back slash is an escape character to flag the next character as a special code. In this case "\v" is the code for a Vertical Tab. So, I had created a connection string looking for a server named "(localdb)VerticalTab11.0".
What made this little error a bit painful was that in this class I had mentioned escape characters in C#, and how to deal with them, at least four times! Oh well…
To solve the problem, escape the escape character ("(localdb)\\v11.0") or mark the entire string as a literal string with the At sign ("con.ConnectionString = @"Data Source=(localdb)\v11.0 …").
For a list of the C# escape characters see this MSDN article: