1. Lack of Contrast
QR codes must have high contrast to be readable. The foreground (dots) must be significantly darker than the background. Never invert a QR code (white dots on a black background), as many older scanners cannot process inverted codes.
2. Quiet Zone Violation
The "Quiet Zone" is the mandatory blank space surrounding the QR code. If text or graphics intrude into this space, the scanner cannot locate the corner alignment markers.
3. Too Much Data
If you encode a massive block of text, the QR code pattern becomes incredibly dense. Dense codes require high-resolution printing and high-quality cameras to scan. Keep URLs short (use a URL shortener if necessary).
4. Printing Too Small
The absolute minimum size for a QR code on a printed asset (like a business card) is 2cm x 2cm (0.8 x 0.8 inches). Anything smaller will fail on older smartphones.