If you're familiar at the shop, they will likely exchange it for you with no problem. I buy a lot online, but I also buy a lot from Hancock's and Wal-mart (before they dropped bolts). I've always made it a practice to get to know the ladies and I've returned fabric a couple of times without a reciept - no problem. However, the big superstores (Joann's, in my area) might not be so accomodating. Online stores have a record of all sales.
In any event, I'd certainly let them know about the bad fabric, even if you got no satisfaction - you might run into a clerk who has managed to hold onto her customer service skills.