Both Visual Studio natively and ReSharper suggest extension methods they found in unimported namespaces from all directly or indirectly referenced packages. There are parentheses showing that they will add a using directive for that namespace if you select that extension method, but if you are moving quickly, you can end up with using Castle.Core.Internal; without noticing. Sometimes the first time you notice is many commits later or after submitting a PR. It's not pleasant to be stepping around these gotcha entries in IntelliSense.