One thing that has been obliquely mentioned so far, which I think deserves a little more attention: if your brain is wired in a particular way -- e.g. enjoying novelty, liking finding and shopping for things -- then strolling through Costco can induce a kind of euphoric stupor, where you are fascinated at all the (presumably!) great deals for items that you didn't even consider you might want or need.
Costco loves this, of course. They foster such euphoria on purpose. The sensation they want to create in the consumer's mind is, oh I am in a warehouse store, that means everything here must be a good deal, therefore if I see anything I'm interested in at all, it is probably not a bad deal to just go ahead and buy it.
I'm not necessarily talking about the (more elementary) scenario of, hey let's buy chicken breast at Costco at $1.99/lb, even though your grocery store has it on sale for $1.69/lb. Yes, watch out for that one too.
What I'm talking about is: hey I came to Costco to buy toilet paper and chicken breast, but WOW here is this giant 2lb bag of dried cherries! I LOVE dried cherries but usually don't buy them much bc they're expensive. But I'm at Costco. So my brain doesn't have normal reference points for what makes a good deal. Because I don't usually shop for dried fruit in a 2lb bag. But it's probably a good deal right? And I love cherries. So I'm going to buy it.
... This circuit can repeat a couple more times if you're not vigilant. Then you come home from Costco having spent an extra $50 or $100 on things you didn't actually need (or even remember that you wanted).
So THAT ^^^^ is what I think is truly the dark side of bulk grocery shopping.
The cure for this is pretty simple, of course: do the same research for Costco that you ought to do at the regular grocery store. Take note of prices over time. PLAN what you are going to buy. Run some math to make sure you're not actually getting taken to the cleaners on that "good deal". Basic stuff. Just easy to overlook, because when you're first getting into it, you can mistakenly believe that anything you buy in bulk is automatically a good deal. False statement.