Back in the old days, you had to send your cartridges out to have them refilled. It could take a week or so. For this reason, two sets of ink cartridges were a good idea: one set in your printer, and one set that you could send out to have it refilled.
My spare cartridges had been lying on a bookshelf in my living room for awhile- probably left out as a reminder that I would need to refill my ink soon. Now I was OUT of ink. I grabbed the cartridges- HP 15 black and HP 78 color, and headed to Cartridge World. They had a color cartridge in stock, so they took my empty cartridge and sold me one that was already refilled. They did not have a black in stock, so they refilled the cartridge while I waited. It was nice! It didn't take long. I didn't have to leave it there and wait a week or two to retrieve the cartridges. Quick, same-day service. PLUS, it saved me about $30 by not having to buy it new.
I went home and tried to install the new ink cartridges. When I opened the printer, I found that my printer DOES NOT take HP 15 & 78, it takes HP 564 (black, magenta, cyan, and yellow).

You can see the obvious differences in the cartridges.
WHAT IN THE WORLD?!
These cartridges look nothing alike. How could I have confused the two? I'll just have to chalk it up to a blonde moment. But a couple of things just don't add up...
I have had those extra cartridges for awhile. I assumed that they were my spare set. I have never owned any other printer. Where did these other cartridges come from? And where was MY spare set for my printer? I just paid to refill cartridges that I can't even use!