art issue
Mac OS 10.10.2 , the shape of right underside corner in finder window is different to the other three
OS X Yosemite (10.10.2)
10.10.2 is not a Classic operating system. Looking at your picture and comparing with my 10.10.2 system that's normal. Actually they are the same shape, it just may be an optical illusion due to the different background contrast. Don't forget each window casts a "shadow" for a three dimensional effect. The shadow reacts differently with different background colors.