Inclination

"The normal inclination is to draw a thick horrzontal line from one of the bar code to the other. It won't work. The lines that make up the bar code are taller than they need to be. If a person draws a horizontal line across them, perpendicular to the individual lines, one has simply changed one tall bar code into short ones, which can still be read.

Instead, some people make a broad, black vertical mark, parallel to other lines, but much heavier. The line needs to be near the center of the code, and broad enough to cover several bars. This is because most bar codes contain redundant information, to permit the bar code reader to double-check its reading. Several lines need to be completely obscured to do the trick. And don't forget that white spaces between lines are also read by the scanner; obscure at least one.

Now, the food container cannot be read by the scanner, and the cashier will be forced out of her normal zombie mode and actually read the label. A person knows how it's done. A person could the same sort of writing utensil, a similar handwriting style, and a discount for out-of-date or damaged merchandise. Each department will have its own style. Some people will not use the butcher's style to mark down the geren eyes.

For it to be effective, it's esential that the person not be seen doing this. Most stores won't arrest a person for it, since it's not a clear-cut case of lareceny. However, the person is likely to be challenged and at least, embarrassed. A person may be photographed and barred form returning to the store."

This is what Andrew said.