Chances are, your "price tag" was an advertised price that was expired. So you paid the right price. Some stores have a strict "tag price" policy. This means that no matter what the product's actual price is, if it was sitting somewhere with a lower price advertised (even if the product description didn't match or the date of the tag was expired) it must be honored at the front. This usually depends on the assistant manager on duty or the store manager's opinion and how hard he enforces it. This being said, I have some doubt that the clerk would've just said "OH" and ignored you. He may have said "OH" and after finishing ringing the customer he was helping, had intentions of calling a manager up to fix the problem. Chances are, given your statement about family waiting for pancakes, upon hearing his "OH" muttering, you promptly turned around and walked out without asking him to refund.
I do agree that he should've apologized or offered the refund. However, you are somewhat contradictory when you say you think that "it's sad that the stores don't even know that these employees are giving them a bad name", yet you are affecting the whole store by not going back due to this one employee. Maybe you should go back and go to a different counter to check out. Or perhaps call the store and actually try to fix the problem you think the store has on its hands.