Sorry in advance for not reading the whole post, but regarding the OP:

I had the same problem on my Macbook. Somehow the Super-key is screwed up and xfce thinks it fires all the time, and <Super>p is a shortcut in xfce. The other one that will annoy you is <Super>tab. Just clean all lines in
Code:
~/.config/xfce4/xfconf/xfce-perchannel-xml/xfce4-keyboard-shortcuts.xml
that contain Super and you should be fine.