Most recently, I just needed a phaser-- specifically, an Electro-Harmonix Small Stone. I walked in, walked up to the counter, and asked for a Small Stone. The clerk looks at me blankly. "Who makes it?"
I answer "Electro-Harmonix."

"You mean a Small Clone?" He responds, naming that company's chorus pedal.
I try not to rankle at his correcting me: "No, I mean the Small Stone." Anyone who doesn't play, and play through pedals, might not get why this annoys me: the EH Small Stone is probably the second most popular phaser pedal in the world, not quite as ubiquitous as the MXR Phase 90, but it's been in production non-stop since the mid 1970s. Someone working behind the effects pedals counter at a music store should at least have heard of it.
He looks up this strange and unfamiliar pedal in his computer. "Hmm... I don't have one in stock. I could order it from a warehouse and give you a call when it comes in." And this is the primary problem with Guitar Center-- if I wanted to order one, I would have done so online, without dealing with the fluorescent lights and the ambiance of someone practicing Metallica riffs on an out of tune guitar: that's what I use the internet for. I walked into the mega-chain-store because I wanted to exit the store with an extremely common pedal in hand.
I've made this mistake more than once-- I have often gone to Guitar Center because they're a big chain, and I expect them to be well stocked. I don't enjoy being in the store, I don't like shopping there, but their size always makes me assume that if I want to find something that everyone uses, something mass produced, this will be the obvious place to go... and I am always wrong: I leave empty handed, wondering why I had to describe a Big Mac to a McDonald's counter jockey (you mean Quarter Pounder? No? Maybe I can order you one from the store across town)