I've switched to Mac at the beginning of 2008 and i since then i found
this bug, but never understood how to reproduce it .. until now.

It happens in all panels with folders which can be moved (so it happens in
Contact, Bookmarks and Notes, but not in Mail): select a folder with the
mouse, mouse gesture "right click + left", now try to select a different
folder: both of them are highlighted and the first is moved into the
second one. Click on a different folder and it happens again.
To "exit" you have to click on an empty area in the panel and it starts
working again.

I've looked for this bug newsgroups, but haven't found anything similar:
is it only in my system?

Cheers.