I believe, Attachment is one of the bigest stumbling blocks for the practice of Zen. Attachment in its many forms may be so deeply rooted in ones mind, as to be totaly unaware of its implications. Any bias, any predjudace no matter how small is attachment. In fact, even thinking that I am unattached, is by that very nature, attachment, I am attached to being unattached.
Well that should get the ball rolling, let's see what subtle forms of attachment anyone else can come up with.