What to do when you've forgotten your bags
Let's face it, sometime you just find yourself walking into the grocery store and your bags are back at home. When this happens, I improvise.
Since all the big grocery stores in my area now have recycle bins for bags, I take a few out and use those, looking inside first to be sure they are acceptably clean.
Also, since the reusable bags now sold at the store have been just a dollar, I sometimes buy one when I've forgotten just to increase my collection right when I need it. If I get too many reusable bags I can always pass them on to someone else.