Customer Service Discounts

This may be old news for some people, but I'd like to share an experience I had at Zellers recently...

I bought something that I thought was on sale (55% off). When I got to the parking lot and reviewed the receipt, I noticed I hadn't gotten the discount. I went back to customer service and was told that the item in question was not actually included in the sale. The customer service cashier said to me:

"Would you like to return it, or do you still wish to purchase it? The best that I can give you is 25% off."

(Really??)

Me: "Well... hmmm. Ok, I guess, thank you, I'll take the 25% off."

I am wondering now, if I'd caught the mistake at the cashier instead of going to customer service afterwards, would she have been able to offer me the 25% discount? Would it have been full price, or not purchase?

I'm thinking that (assuming you have the time) that instead of checking items as they are scanned at the till for sales discounts, you're better off waiting until transaction is paid for and closed, and then going to customer service with any discrepancies.

Has anyone out there had a similar experience, or any insight to share?

J