Yesterday at Walmart I checked my receipt and thought I was overcharged for my haircoloring - so I left the buggy with DH and checked the shelf. Well, the price had gone up - but not on my color! They gave me back $8.56 - well worth the bother.
Sometimes I write the price on the items (I always carry a pen) and then it is easy to check as items are being scanned.
My GF takes a picture with her cellphone to prove the prices at the register.
Whatever works, money in your pocket!