A Project Item with the following item ID could not be found

I got this error message from Visual Studio 2010 today when I was building a SharePoint package:

“A Project Item with the following item ID could not be found: 3725bd89-3a48-4333-84dd-c99d25cc1904”

Say what?
I searched through my solution for that ID and came up with nothing.

So what now then?

I started to retrace my steps because I had previously had no problems with deploying/packaging the SharePoint project.
I had recently added a new mapped folder to my project which mapped to the resources folder in the SharePoint root directory and in that mapped folder I had added a new resource file. Could that be the culprit?

The error message states that a project item (SPI) could not be found so I thought I´d have a look at the Package.package(xml) file outside of Visual Studio to see if that file contained anything suspicious:

OK, so there is that ID that is causing all this problems. But I can´t understand why?

I restarted Visual Studio and look at the project structure:

There is no “Resources” mapped folder in there. I guess something happened when I added that folder to Visual Studio and now it is not included in project structure anymore.

Now looking in the packaging explorer there is also an error. I removed that item and re-added the “Resources” folder again through the “include in project” menu option:

And now…yes it works again.

Is this a bug or did I do something wrong?
I don´t know but for now I´ll leave it at that and if I encounter this issue again I´ll always start with restarting Visual Studio to see if that gives me any ideas…