I can only think of one note that is square or not a rectangle in my collection.

It did not start out life this shape, as it would have had 8 coupons attached to it (4 per side) measuring roughly 150mm T x 295mm W, but as the coupons were removed it was reduced to a lowly square. Totally L7 man. 

@Fenntucky Mike I would class that note as 'square' ! Great detail and an essay to read whilst you consider spending it.

There seem to be a few others about, odd ones from various countries but most square ones I have seen appear to be mainly early French and Dutch issues. I think the kids would just confuse them with post-it-notes.