There was this "fake window" with the desktop icons on my desktop. I hope you know what I mean by "fake window" (I don't know the official name of it). It's the thing where the icons are that in most window managers are on the desktop.

I dragged this thingie from the left screen to the top of the right screen, and suddenly it disappeared, as if it warped somewhere outside of the screen coordinates. And I can't get it back!

What can I do to get it back in view? I think it must still be there... Unless maybe someone can confirm that it closed itself when it disappeared?

It's called "widget", and the widget name is "Folder View".
You can add a new Folder View to your desktop. Which KDE SC do you use. For 4.3.x:



The procedure is similar in 4.4.x, but the "Add Widgets" explorer looks different.

The disappearance thing is probably a bug (you shouldn't be able to lose widgets like that). If you can reproduce the issue, could you please file a bug report at bugs.kde.org (after first doing a search to avoid duplicates) and post relevant information, such as
- Steps to reproduce
- Your setup (dual monitors? twinview? KDE SC version?)

and finally post the link to the bug here.

I saw a similar thing once. When I zoomed out I saw that it had moved to the next activity! If that's happened you can drag it across to the correct one, but it'es easier to just delete it and create a new one as Hans said.

Right click on your desktop and select the option that takes you to the place where you can set your desktop to be "folder view". Having a desktop in a folder is sort of contradictory to common sense, as it is afterall your desktop. IMHO putting a desktop in a folder makes no sense to me.

Desktop always was a folder, as was obvious in the file manager. YOu just had a special view directly onto it, which basically is what the folderview desktop does.