Aisle Be Seeing You: Checkout Lane Ethics

So you're in line at the grocery store and the cashier is just starting to ring up your groceries when you realize you've forgotten something. Is there anything wrong with running back and grabbing it if there is someone behind you? Are you right in being annoyed if the person in front of you does this? Is there a difference between forgetting something and running back for it and getting a spot in line with the intent of finishing your shopping while in line? By getting in line, have you tacitly agreed to have finished shopping?