1.) Place two barrels side by side, take off the lids
2.) Open inventory, most of the time at least one of the barrels' inventories will render so that only the far left edge is visible

The issue can be fixed by opening and closing the barrels in different combinations until you get lucky. Sometimes you can click the "inventory" tab (the square with an X in it) to make it hide then click it again and it will render correctly.