http://docs.unity3d.com/ScriptReference/WWW-error.html

In the description and code examples "String.IsNullOrEmpty" is used, however this is correct only for the JavaScript code. In C# it should be "string.IsNullOrEmpty". I am not sure if this should be changed in the description, however this mistake must be fixed in the C# code example.

Additionally all code examples (in JavaScript, C# and Boo languages) have a mistake, because the www.texture result is assigned to the texture even though an error was returned. The assignment should be controlled by the "if" statement.