So I went shopping for my friend's DS first birthday. I bought a few little outfits in M&S, I was vaguely aware that I'd spent maybe £70. Added some food bits in as well, went to the till.
I watch the guy ring everything up, carefully taking things off hangers etc and scanning them all. Came out at £58. Ooh, I must have added wrong, I thought.
Headed home, wrapped them all up, put the receipt away in case something didn't fit. Handed gifts over.
I was thinking about it later and thought, I'll check the receipt. A whole outfit wasn't scanned. 
AIBU not to go back and explain/pay? Maybe this should be in WWYD - I feel guilty but it's five days later now!