Last night while developing an event listener custom list, I ran into this error,

SharePoint Error: “The language-neutral solution package was not found”

The solution to this problem is, Restart your visual studio. And it will be back to normal.

Hope this helps

The .Net Framework has a feature called Shadow Copy.

Shadow copy is enabled on every appdomain created by ASP.NET by default. By default assemblies loaded will be copied to a shadow copy cache directory, and will be used from that location.

But this may create problem sometimes.