One takeaway is that disrupting/breaking QRs with a minimum of edits (and with only a sharpie) is a _lot_ easier than adversarially modifying one to data under your control. The crucial format bits which specify the code's error correction level and mask pattern is only 15 bits (5 data + 10 ECC). It appears in duplicate in the code, but many of those 30 bits will be dark already and you can black out the rest. As the layout of the bits is linear, it's easier to black out multiple bits at once.