Our test server got this Exception a zillion times when we did our performance test:
System.Data.SqlClient.SqlException: An error has occurred while establishing a connection to the server. When connecting to SQL Server 2005, this failure may be caused by the fact that under the default settings SQL Server does not allow remote connections. (provider: TCP Provider, error: 0 - Only one usage of each socket address (protocol/network address/port) is normally permitted.)
Unfortunately Google as of today reveals very little information about this. To make a long story short this is created by one of the guys mistakenly adding the connection string with "Pooling=false;". This caused a humongous amount of connections, probably flooding the network and popping up the exception.