.box-N's should use "float:left" like .span-N's

[BradBuckingham]

Adding

 .box, .box-1, .box-2, .box-3, .box-4, .box-5, .box-6, .box-7, .box-8, .box-9, .box-10 { float: left; }

to gutterless.css will make sure that any .box-N's are contained within any span-N.