In Dan Jacobson's photo of the MT/ND/SK tripoint 
(
http://www.geocities.com/jidanni/images/19940721skndmt.jpg), the old 
U.S. Customs warning sign appears (to me, anyway) to be placed 
slightly north of the international boundary monument.  Optical 
illusion...or did the US actually place this sign on Canadian soil?  
(Dan's notes indicate that the photo was taken looking SSW, which 
seems to coincide with the angle of the MT/ND boundary).
Bill