Extra using statements will very rarely cause problems, and when they do they will be because you have introduced an ambiguous name into the scope (for example,
using two namespaces with both define a
Vector type, and then trying to write
Vector myVector = new Vector() will be ambiguous). The compiler will tell you of these name clashes, and fail to compile your code.
If you run afoul of this, you can either fully-qualify the name reference (
SomeOtherNamespace.Vector) or remove the unused
Note that if you are using Visual Studio, there is an option in the right-click menu while in the text editor for a source file to "remove unused usings." You can also have them sorted via a similar menu command.