Autostart is supposedly expecting only executable files. If a non-executable file is copied to ~/.kde4/Autostart  it tries to open whatever application the file type is associated with.
In my case, it was a text file and it tried to open Kate, which would never finally open.
The thing is, the desktop froze. The plasma panels would not respond at all. I tried to open Rekonq and it wouldn't open.

I had to hit Ctrl+Esc, kill the Kate process. Then a Kdeinit error showed up, and all other stuff opened, and the desktop was responsive again.

My guess is, if the file in Autostart is not executable, bad things may happen and it shouldn't try to execute it at all, or verified that it doesn't freeze the desktop if it fails somehow.

To reproduce, it should be just enough to place a normal text in ~/.kde4/Autostart and relogin.