I can't put into words how irrationally angry I get when the "Click for larger image" button on a product website opens up a bigger window with an image the exact same size as the original.  As a casual programmer, I can attest to the fact that this problem is stupidly easy to fix: 

if (largerimagesize > currentimagesize) {
   display_link();
} else {
   do_nothing();
}

Internet, fix it!