Checking out of the grocery store is definitely a time you need to be paying attention and watching what is going on. Week after week there are register errors that cost consumers lots of money! Whether it is the cashier accidentally scanning an item twice, or a sale item coming up at regular price, you need to be on careful watch to point out mistakes and have them correct them.
There are many stores that have price accuracy guarantees that offer dollar incentives or free items if the price comes up incorrectly. Check your store for their policies... most won't volunteer this information or say anything unless you point it out.
Finally, never think an amount is too little to point out. Add up those little mistakes over a year and you could be looking at substantial savings. :)