Good info to have, but where do you find out these things? (Does it go along with the double secret handshake?)
First I've heard of these useful pieces of info and I've been a Costco member for 17 years IIRC.
I recently bought an item at Costco that didn't have a listed sale price but had a very significant discount at the register. On that basis, I went back and got more and THEN found out, the discount was only for 5 each (full price for more than 5.) NOTHING was spelled out on the listed price. I bought more later and, sure enough, the sale was still in effect (with the 5-only limit) and STILL no indication on the posted price. I'm talking about a couple of weeks or more here. Eventually (maybe a month+) the price went back to the posted price.