The Devil is not in the details
The myth that UPC barcodes contain the number 666 is an oldie but goodie. It contains some perfect elements for a great urban legend: something ubiquitous to everyday life actually holds a dark secret! It hints at a shadowy cabal of the powerful disseminating that dark secret without our knowledge. And it's pretty tough for a layperson to dis/prove.
Unfortunately for conspiracy-hunters, it’s just a funny superstition. One that likely began with an admixture of paranoia that barcodes could be used (by the aforementionaed cabal, or perhaps the government) to brand and track individuals, and Revelation 13:16-18 from the bible.
16 And he causeth all, both small and great, rich and poor, free and bond, to receive a mark in their right hand, or in their foreheads:
17 And that no man might buy or sell, save he that had the mark, or the name of the beast, or the number of his name.
18 Here is wisdom. Let him that hath understanding count the number of the beast: for it is the number of a man; and his number is Six hundred threescore and six. ~ KJV, 2000
Some ambitious conspiracist did a little research and declared that the left, middle, and right guard bars in a UPC barcode are actually numbers - the number 6, natch - and that's where Lucifer himself lurks. UPC guard bars, however, are just reference points that scanners use when decoding a barcode. The guard bars don’t encode any data: not even '6'.
So now you know. If you still prefer to believe that UPC barcodes contain 666 you can always play it safe by never, ever buying anything with a UPC barcode. Stay safe out there!