I used to think that this error was really fucking stupid, until I realised that there is in fact a valid reason for it.

If you drag an object onto a taskbar button, Windows has no way to know which part of the window you wanted it dragged onto.

Of course, that's no excuse for Microsoft's failure to explain why you can't drag something onto a taskbar button. Something in the help, or perhaps a hyperlink from the error message itself would have sufficed.

More sensible would be to provide a way for applications to specify a default drop target for this situation.

I still think Windows is really fucking stupid in lots of other ways.