When you create your Visual Studio extension, whether an add-in or package, likely you test it most of the time against project types provided by Visual Studio out of the box, such as VB.NET/C# projects, smartdevice projects, database projects, etc.

Long time ago I wrote an article warning that likely add-ins fail throwing exceptions when tested against 3rd party project types, because they tend to be “unmodeled projects” (meaning that they don’t support automation):

PRB: Add-ins fail with 3rd party project kinds in Visual Studio .NET
http://www.mztools.com/articles/2004/MZ2004006.aspx

New project types can introduce new icons for project/project items, new text file types that are searchable, etc. so your add-in can be affected.

While you may argue that it is annoying and time-consuming to test an add-in/package against every single new project type that any 3rd party company feels the need to create, there are new project types from a company that you should test: the new project types from Microsoft itself. This is because they are extensions for Microsoft products and technologies and very likely they will be installed on tons of computers with Visual Studio. My latest article mentions some of the new extensions or tools for Visual Studio that you should test against:

INFO: Testing a Visual Studio add-in against new Visual Studio tools or extensions from Microsoft
http://www.mztools.com/Articles/2009/MZ2009003.aspx