I have a button that wants to move around when trying to be clicked. It's different on other computers, but always an issue with at least one product.
If you go to my site: http://www.redheadcreamery.com, then store>products>click on kids tshirt and try to add it to the cart... for my husband's computer, it was the men's tshirt. Didn't matter what size.
The 'Add to cart' button will shift down a line and won't let you click on it.. unless, you hold down your courser to follow the shift and then deselect. I don't expect my customers to know that though.