I have an HP Office Jet K60 All-in-one printer. One of my relatives has the previous version of the same multifunction machine, which uses the same cartridges as mine.

I purchased a new HP color cartridge about 3 weeks ago. I haven't gotten a low ink message, but all of a sudden, when I printed a color page from the internet some of the ink looked like it does when it is running out.

I suspect my relative of switching her used cartridge with my new cartridge. I still have the box the new one came in and it has a number on one of the flaps. Is this a serial number, and is it also marked on the cartridge?

I would like to know if I am just being paranoid, or if there is some other problem.

Each cartridge is marked with a serial number, but it is probably not the serial number for that individual unit. That serial number probably refers to that type of product; the brand, the size, the type, etc. Two of those types of cartridges would have the same serial number.