You would be amazed, or probably NOT so amazed at the number of times an item rings up incorrectly at the cash register. It happens to me more often than not.
I was reminded today at Wal Mart as I glanced at my receipt as I walked out the door. Those two Land O'Lake Boxes of Butter I bought with a rollback price of $1.00, actually rang up at $1.28 . I also purchased some Feta cheese for $1.88 but rang up at $2.98 . Yikes! Third time is a charm with pizza on the shelf at $4.50 but rang up at $4.75 . It doesn't seem like much, but in the long run it really adds up.
Checking your receipt after the fact is the next best thing if you forgot to watch as the items were scanned, as I just recently went through the line at Publix with one bag of salad at $3, and yet somewhat the same one bag was scanned 3 times. Luckily I examined the receipt, and made a return trip to Publix to get my $6 back, which sadly lacked an apology for their error.
In a search, I found this article at wakeupwalmart.com, it's a few years old I think, but appears nothing has changed in 2009 with scanning errors.